Are we asking the wrong questions about dark matter?

Whenever the topic of dark matter comes up, the most common question that people wonder about it is whether this mysterious, hypothetical substance really exists, and whether it truly dominates the matter content in our Universe as thoroughly as its adherents say it does. Some advocate for a position where dark matter doesn’t exist, and that there’s just a large amount of normal matter that’s unseen to hold the Universe together. Others advocate for a scenario where there’s only a “normal” amount of normal matter alone, and that instead of dark matter to provide the needed gravitational glue to hold structures together, a modification to the laws of gravity will do the trick instead.

But those advocates are an extraordinarily slim minority of working scientists: a group of contrarians whose claims fail to explain the overwhelming majority of evidence. Dark matter’s existence is overwhelming accepted by astrophysicists and cosmologists who work with the actual Universe as we observe it, with the alternatives all failing to match those observations without invoking either some form of dark matter or something that behaves identically to it.

Instead of arguing over the existence or non-existence of dark matter, the community has largely moved on to the next step: attempting to characterize dark matter, and to figure out what type — or flavor — of dark matter actually exists within our Universe. Here’s the story from the cutting edge that everyone should know.

Our galaxy is thought to be embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter flowing through the solar system. Although we have yet to detect dark matter directly, the fact that it’s all around us makes the possibility of detecting it, if we can surmise its properties correctly, a real possibility in the 21st century.

It’s important to recognize that there’s an enormous suite of evidence in support of dark matter, observationally, that no alternative can explain equally well.

• There’s Big Bang nucleosynthesis, and the abundance of the light elements, that tells us that precisely 4.9% of our Universe, energetically, is composed of normal matter, and no more.
• There are the fluctuation patterns in the CMB, which reveal a dark matter-to-normal matter ratio of 5-to-1, and a Universe that’s about 27% dark matter, with just 5% normal matter.
• There is the large-scale clustering data of the Universe, as well as the baryonic acoustic oscillation feature within that large-scale structure, that reveal that same 5-to-1 dark matter-to-normal matter ratio.
• There’s gravitational lenses, that reveal the total masses of galaxies and clusters and find that it’s far, far greater (by about a factor of 5-6) than normal matter alone can account for.
• There’s the existence of colliding galaxy clusters, that clearly reveal a separation between the distribution of normal matter and hot, X-ray emitting gas from the overall gravitational effects, thought to be primarily due to dark matter.

And there are many other lines of evidence as well that all support dark matter’s existence, as this one additional ingredient in the Universe can sufficiently explain them all. Sure, we haven’t directly detected the underlying particle responsible for dark matter (assuming there is one), but there’s no guarantee that we’ll be able to do so, even in principle. The observational evidence, even though it’s indirect, is sufficiently overwhelming to convince all but the staunchest contrarians that dark matter does indeed exist.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

The next step, then, is to try to use all of the astrophysical tools at our disposal to try and determine what type of dark matter exists in our Universe. This dark matter must have been around very early on in cosmic history: back in the primordial plasma stage of the hot Big Bang at least, or it wouldn’t have imprinted itself in the CMB and large-scale structure the way observations indicate. Back in those early stages, dark matter must have had an initial energy to it — a form of kinetic energy — and a mass to it as well. Astrophysicists generally divide dark matter into three possible categories based on the relationship of its kinetic energy to its rest mass.

1. Hot: this would be dark matter that was light in mass but high in kinetic energy, which is a type of dark matter that moved relativistically, or close to the speed of light, for much of cosmic history. This would suppress the formation of small-scale structure and lead to shallower gravitational potential wells on small cosmic scales.
2. Warm: this would be dark matter that has a modest amount of initial kinetic energy relative to its mass, where it moved relativistically early on, but then cooled to move non-relativistically at intermediate times. It would lead to steeper potential wells on small scales than hot dark matter scenarios, but shallower dark matter halos than if dark matter were non-relativistic, or cold.
3. Cold: this would be dark matter that was either very massive, so that it’s kinetic energy was unimportant early on, or dark matter that was light but born non-relativistically, so that it was able to clump and cluster even at early times. This leads to the steepest gravitational potential wells, and does so even on small cosmic scales.

The difference between these three scenarios shows up most glaringly in the large-scale structure of the cosmic web.

The dark matter structures which form in the Universe (left) and the visible galactic structures that result (right) are shown from top-down in a cold, warm, and hot dark matter Universe. From the observations we have, at least 98%+ of the dark matter must be either cold or warm; hot is ruled out. Observations of many different aspects of the Universe on a variety of different scales all point, indirectly, to the existence of dark matter, but direct detection experiments haven’t found the particle responsible for it.

Just from the clustering data, we can rule out the hot dark matter scenarios, and most of the warm dark matter scenarios as well. But there are even better avenues for not only telling hot, warm, and cold dark matter apart from one another, but for investigating some very detailed properties of dark matter that appear, cosmically, in a model-dependent fashion.

Sure, most simulations make the simplest assumption of all: that dark matter is a cold, collisionless, pressureless entity, that only interacts gravitationally and has no (or negligible) self-interactions and no (or negligible) interactions with normal matter. That’s sort of the “spherical cow” version of dark matter: where you make a grossly oversimplified, even unphysical set of assumptions just to get at the most rudimentary approximation of an answer.

Those types of simulations have formed the basic skeleton of our framework for the idea of dark matter halos: or large, spheroidal distributions of dark matter that are incredibly diffuse and fluffy. Dark matter halos, in the broadest sense, will have a density profile, and some of the common profiles that arise in these types of simulations are the Navarro-Frenk-White profile (or NFW profile, for short), the Moore profile (after Ben Moore), and the isothermal profile, which assumes some type of interaction, rather than the more typical non-interacting cold dark matter scenarios.

Four different dark matter density profiles from simulations, along with a (modeled) isothermal profile (in red) that better matches the observations but that simulations fail to reproduce. Note that these dark matter profiles occur with the same slopes but different turnover radii on different cosmic scales, and do not include the effects of gas and stars, whose roles have been (perhaps unfairly) neglected.

But there’s more to the story than that. We typically assume that the Universe was, way back in its early stages, in an approximate condition known as thermal equilibrium: where every particle that was present had so much kinetic energy, and where there were so many particles of such great densities in such a small volume of space, that collisions were frequent. Each time a collision occurs, energy gets transferred between the colliding particles. In very short order, then, particles wind up taking on the same energy distribution as all other particles around them, even across particle species.

If this is true for dark matter, too, then even if dark matter is cold, it should have, imprinted on it, the same initial spectrum of density fluctuations on all cosmic scales that all other particles inherit: as a leftover relic of cosmic inflation that set up and spawned our Big Bang. That means that, dependent on the mass of dark matter and the time period that it decouples from the primordial plasma (i.e., stops interacting and colliding), it won’t just be a smoothly varying density profile of a dark matter halo that surrounds galaxies, but rather there will be a distribution of halos of all different sizes and masses embedded within every large dark matter halos.

In other words, every halo will have large numbers of smaller halos of lower masses embedded inside of them: an ocean of complex dark matter substructure within each and every large dark matter halo.

This Hubble image of massive galaxy cluster MACS J1206 shows the arcing and smeared-out features caused by the gravitational light-bending of the foreground galaxy cluster. Small-scale concentrations of dark matter, represented in blue, have been reconstructed based on the lensing data. Combining this lensing information with intracluster light information, which is another, independent tracer of dark matter, can reveal its presence and distribution as never before. We discover, through analyses like this, that all dark matter haloes consist of a rich set of dark matter substructures.

So, how can we probe these dark matter halos for this substructure? How can we use observations that we can actually make, of real tracers that are actually in our Universe, to determine things like the depth of potential wells in dark matter halos, the presence and mass distribution of dark matter substructure, and to map out the density profiles of the massive halos that surround galaxies and galaxy clusters?

It turns out that there are four main methods we can use, and there are many people working on leveraging all four to try and answer these questions. If we can obtain answers that are good enough, we’ll be able to rule out scenarios for dark matter that don’t match the data, while providing insights into which models of dark matter — and which cosmological simulations based on those models — are favored by observations of the actual Universe.

1.) The Lyman-α forest. When you have distant sources of very bright light, such as from a quasar, an active galactic nucleus, or an extremely bright galaxy, that light doesn’t just come across the Universe to our eyes. Instead, it passes through all of the intervening matter from there to here: clouds of gas, galaxies large and small, and even potentially “dark objects” that have matter in them, but no stars. Each time any sort of matter is encountered, as long as there are neutral atoms inside, part of that light, at very particular wavelengths (including the transition between the two lowest energy levels of hydrogen), will get absorbed.

Distant sources of light — from galaxies, quasars, and even the cosmic microwave background — must pass through clouds of normal matter. The absorption features we see enable us to measure many features about the intervening gas clouds, including the abundances of the light elements inside and the degree of ionization. Even starless clouds of gas and dark matter will have absorption features that embed themselves in the spectrum of the background light: part of the Lyman-α forest.

Credit: Ed Janssen/ESO

That leads to a feature known as the Lyman-α forest: a “forest” of lines where each cloud of gas is responsible for one of the absorption lines that represents a single “tree” in that forest. Already, the Lyman-α forest provides the strongest constraints on scenarios for hot and warm dark matter in the Universe today, and the depth of each line at each particular distance helps us understand how small-scale and medium-scale structure, driven by dark matter, grows and evolves over cosmic history. This method will no doubt continue to bear fruit as new wide-field observatories, like Vera Rubin, Euclid, SPHEREx, and Nancy Roman all take data.

2.) The intracluster light. Galaxy clusters are some of the messiest environments in all the Universe, with thousands of large galaxies, quadrillions of solar masses worth of material, millions of globular clusters, and the most giant, enormous, massive, extended dark matter halos found anywhere in the cosmos. Within those giant halos, however, are enormous numbers of smaller sub-halos, not just around each of the individual galaxies within them, but on even smaller scales. When normal matter gets absorbed from small galaxies into larger ones, or pulled into giant ellipticals, or gets cannibalized by larger neighbors, the initial dark matter halos persist.

And, as stars get ripped out of galaxies or form as the result of tidal interactions in the intergalactic (but intracluster) medium, they eventually, over long enough timescales, begin to follow and trace out the overall distribution of matter within the cluster itself.

This image showcases the massive, distant galaxy cluster Abell S1063, imaged as part of the Hubble Frontier Fields program. The diffuse, bluish-white light shown here is actual intracluster starlight, which was captured for the first time only in 2018. This light at least partially arises from material stripped out of galaxies within the cluster that have collapsed to form stars. Those stars then follow the cluster’s overall gravity, tracing out the underlying dark matter distribution.

This means that the intracluster light that we observe within clusters is actually a tracer of dark matter: both of the overall interior distribution of dark matter, including its density profile, and also of the various dark matter sub-halos that exist within the intracluster medium. Measuring the light gives us the means and ability to trace out the dark matter, and tracing out the dark matter, to greater spatial resolutions and down to more sensitive brightness degradations, means that we can gain insights into the substructure of dark matter within these cosmic behemoths. If we can examine these clusters at great distances, we might even come to understand how dark matter halo and sub-halo distributions evolve over cosmic time.

3.) Using strong gravitational lensing in extremely sensitive configurations. In Einstein’s general relativity, clumps of matter and energy don’t just cause spacetime to curve, they can cause it to behave as a lens: distorting the light from background objects behind it. While weak effects include altering the shape and location in the appearance of background objects, the right geometric configuration and the right mass distribution, combined, can lead to extreme effects: the stretching and magnification of background sources, and in some cases, the appearance of multiple images, where multiple separate light-paths all result in light from a background source arriving at our eyes.

This image highlights six of the eight quadruply lensed systems first used to place the greatest model-independent constraints on dark matter’s temperature and mass from structure formation. These images have also revealed the presence and distribution of dark matter substructure within the foreground lens system for each such system that was investigated: one of the most powerful probes of dark matter on small scales.

Some of the most spectacular gravitational lens examples are quadruply lensed sources, also known as an Einstein cross configuration. These quadruple lenses, in particular, are not just sensitive to the overall mass and density profile of the foreground mass that’s acting as the lens, but are sensitive to even small perturbations due to dark matter substructure within these objects. With large surveys ongoing at high resolutions, the number of quadruple lenses is expected to explode, and with it, our understanding of dark matter substructure within galaxies and galaxy clusters: whatever it is that happens to be acting as the gravitational lens with four multiple images in these cross-configurations behind them.

4.) Using stellar streams to reconstruct the gravity of the environments that created them. Here in our own Milky Way, we don’t just have stars located in the disk of our galaxy, in the central bulge, and more diffusely in the halo of our galaxy, although those are all present. We also have globular clusters and satellite galaxies: some of which are in the process of being tidally stripped and/or torn apart by interactions with the Milky Way itself. When tidal interactions are strong enough — i.e., when the difference in the gravitational force on the “near part” and “far part” of the object are sufficiently large — the entire galaxy or globular cluster itself can be destroyed, stretched out into a stream of stars that go through or around the larger parent galaxy: in our case, the Milky Way.

This illustration of our Milky Way shows an ancient galactic stream wrapped around our galaxy’s plane at nearly a 90 degree angle: evidence for a recent and even ongoing merger in our galaxy’s history.

It turns out that these stellar streams, when we look at their detailed properties, are incredibly dependent on the dark matter distribution within the galaxy that tore their parent bodies apart! When multiple streams are present, and when they’re present at different radii relative to the galactic center, they can provide insight into how the dark matter distribution of the galaxy varies with distance and also on how much mass is locked up in these clumps within smaller halos embedded within the main one. In fact, the recent discovery here in 2026 of the first globular cluster stellar stream outside of the Milky Way gives us hope that large, detailed surveys will make this a method for cosmically reconstructing the presence of dark matter substructure on galactic scales of all different varieties.

It might be fashionable to rehash old debates about whether or not dark matter actually exists, but this is realistically an issue that’s been settled for decades, and has long ceased to be of general scientific interest to those working in the field. Instead, the current frontier is focused on how we can use direct observables to gain insight into the nature, species, and properties of dark matter, and to see which models pass (and which ones fail) these various cosmic tests. As better data comes in from our current generation of cutting-edge observatories, we should be able to understand dark matter’s nature better than ever before, closing in on precisely what it can and cannot be, even without a successful direct detection via laboratory experiment at all.

This article Are we asking the wrong questions about dark matter? is featured on Big Think.