Scientists are explorers by nature, and when the edges of their maps are terra incognita, researchers sometimes must give names to phenomena for which we have little knowledge. Sometimes those names linger after we know exactly what an unknown quantity is; X-rays are a classic example. The "X" initially referred to mystery, but by the time physicists determined they were simply a high-energy form of light, the name had stuck.
Dark matter, however, is still a placeholder term. Over the decades since astronomers determined that most of the mass in the cosmos is invisible, researchers have done a much better job of figuring out what dark matter isn't than what it actually is. We know it must be electrically neutral, and it can't be made up of ordinary matter (electrons, atomic nuclei, etc.). And while "dark matter" itself is a general term, physicists have a sort of cartography of hints: areas on the map in which various dark matter candidates reside.
The most popular of these realms contains the WIMPs: weakly interacting massive particles. Like the term "dark matter," WIMP is generic: the name describes the energy scale at which these hypothetical particles interact with ordinary matter, which in turn reveals something about their mass.
WIMPs have an advantage from a theoretical standpoint in that particle physicists have some idea of how they could have been created in the first moments after the Big Bang. Beyond that, though, multiple theories can produce particles that look like WIMPs, including versions of supersymmetry (SUSY). For that reason, the majority of dark matter experiments are looking for WIMPs, and in most discussions when we say "dark matter," we really mean "WIMP."
But WIMPs aren't the only spot on the map that looks promising. In fact, the failure of dark matter experiments to find any particles so far could mean we should look to other locales. For example, if you read older accounts of dark matter studies, you'll likely see two non-WIMP possibilities mentioned: MAssive Compact Halo Objects (MACHOs, another "clever" acronym) and miniature black holes created in the first moments after the Big Bang.
If the case for WIMPs looks a little shaky, the arguments for these possibilities look even worse. MACHOs such as brown dwarfs do exist, but observations indicate they comprise no more than 20 percent of the galactic halo mass. Similarly, even tiny black holes would leave a cumulative gravitational signature, which we simply don't see. The map in these areas is clear: while MACHOs and tiny black holes may exist, they simply aren't numerous enough to be dark matter.
That brings us back to the most likely possibility: dark matter is made of particles of one or more unknown types. That still leaves us with many candidates, ranging from the (relatively) mundane to the very speculative. The following is a look at a few of these, but the map still has many blank spaces—it's nearly impossible to be comprehensive at this point.
Galaxies don't like it hot
Most of the matter described by the Standard Model of particles and their interactions isn't an option for dark matter. But there's one known particle left that seems to fit the description: neutrinos. These are very common particles, created in a wide number of fusion and fission reactions. Neutrinos are invisible to all forms of light and barely interact with electrons and atomic nuclei, mainly because they are electrically neutral.
From a dark matter perspective, however, there's a problem: they are also very low-mass—less than one thousandth of an electron mass. That means, numerous as they are, neutrinos don't have the numbers to be dark matter. (There's still a small amount of wiggle room for a higher-mass "sterile" neutrino.)
In fact, most very low-mass particles are poor dark matter candidates. Collectively, they're known as "hot dark matter" (HDM), as their low mass ensures that they move very close to the speed of light with the slightest input of energy, and their lack of electric charge means they are hard to slow down once they're moving. For that reason, HDM doesn't aggregate very well: instead of making the nice galactic halos where we know dark matter lives, the particles would have streamed away from galaxies as they formed. If too much matter is in the form of high-velocity particles, galaxies and galaxy clusters can't form at all. For that reason, HDM is no longer considered a viable model.
WIMPs, by contrast, are "cold dark matter" (CDM), meaning they move sluggishly and can collect into galactic halos just fine. A typical WIMP model predicts dark matter particles to be at least 40 times the mass of a proton, possibly up to roughly a hundred thousand times greater—nicely explaining the "massive particle" part of the name. Similarly, the "weakly interacting" bit is a reference to the energy at which hypothetical dark matter particles interact with ordinary matter and each other. Neutrinos and their ilk pass the "WIP" part but not the "M."
The middle ground between hot and cold, as you might guess, is "warm dark matter" or WDM. Hypothetical WDM particles could have masses starting at about a hundredth of an electron mass: much heavier than neutrinos but far lighter than any particle of ordinary matter. (Protons and neutrons are about 2,000 times more massive than electrons.) As such, they would move more slowly than HDM and could clump up into galactic halos.
Some astronomers are fond of the WDM idea because it could help resolve some problems in understanding the structure of galaxies. The simplest CDM models predict various galactic properties that are at odds with observations—the number and distribution of satellite galaxies, for example—and WDM might solve these issues. The problems are complicated, as with anything involving galaxies, so it's premature to say CDM can't do the trick. However, the WDM and CDM models predict many differences we could observe through astronomy.
Distinctions between the two options on the physics side are of potential concern, though. WIMPs, in whichever theory you prefer, are known as "thermal relics"—they were created in the high temperatures during the Universe's earliest moments and have survived unchanged since. WDM particles would have to be made more recently, either by decaying from unstable WIMPs (an option known weirdly as "super-WIMPs") or some other mechanism. They are not thermal relics.
Additionally, WDM is unlikely to interact with ordinary matter through any force other than gravity. That, combined with the particles' low mass, makes it very hard to detect directly.