Based on the evidence, most astronomers agree that dark matter exists. Beyond that, they have more questions than answers. The biggest question, dare we say one of the biggest in all of cosmology, centers on the exact nature of dark matter. Is it an exotic, undiscovered type of matter, or is it ordinary matter that we have difficulty observing?
The latter possibility seems unlikely, but astronomers have considered a few candidates, which they refer to as MACHOs, or massive compact halo objects. MACHOs are large objects that reside in the halos of galaxies but elude detection because they have such low luminosities. Such objects include brown dwarfs, exceedingly dim white dwarfs, neutron stars and even black holes. MACHOs probably contribute somewhat to the dark matter mystery, but there are simply not enough of them to account for all of the dark matter in a single galaxy or cluster of galaxies.
Astronomers think it's more likely that dark matter consists of an entirely new type of matter built from a new kind of elementary particle. At first, they considered neutrinos, fundamental particles first postulated in the 1930s and then discovered in the 1950s, but because they have such little mass, scientists are doubtful they make up much dark matter. Other candidates are figments of scientific imagination. They are known as WIMPs (for weakly interacting massive particles), and if they exist, these particles have masses tens or hundreds of times greater than that of a proton but interact so weakly with ordinary matter that they're difficult to detect. WIMPs could include any number of strange particles, such as:
- Neutralinos (massive neutrinos) – Hypothetical particles that are similar to neutrinos, but heavier and slower. Although they haven't been discovered, they're a front-runner in the WIMPs category.
- Axions – Small, neutral particles with a mass less than a millionth of an electron. Axions may have been produced abundantly during the big bang.
- Photinos – Similar to photons, each with a mass 10 to 100 times greater than a proton. Photinos are uncharged and, true to the WIMP moniker, interact weakly with matter.
Scientists around the world continue to hunt aggressively for these particles. One of their most important laboratories, the Large Hadron Collider (LHC), lies deep underground in a 16.5-mile long circular tunnel that crosses the French-Swiss border. Inside the tunnel, electric fields accelerate two proton-packed beams to absurd speeds and then allow them to collide, which liberates a complex spray of particles. The goal of LHC experiments isn't to produce WIMPs directly, but to produce other particles that might decay into dark matter. This decay process, although nearly instantaneous, would allow scientists to track momentum and energy changes that would provide indirect evidence of a brand-new particle.
Other experiments involve underground detectors hoping to register dark matter particles zipping by and through Earth (see sidebar).