Perhaps dark matter comes in the form of MACHOs, a playful acronym for MAssive Compact Halo Objects. Some possible MACHOs include brown dwarfs, objects smaller than stars but bigger than planets; black dwarfs, old stars that have exhausted their nuclear fuel and are now slowly fading away; and large planets with masses similar to Jupiter’s. After years of searching, astronomers have discovered a handful of brown dwarfs, and recent surveys have turned up a number of MACHOs by observing how their presence distorts and magnifies the light from stars in other galaxies they happen to pass in front of. But at this point, not enough MACHOS have been observed to account for the quantity of dark matter scientists believe is shaping the universe.

       But that’s not to say other forms of dark matter can’t exist. A leading candidate is the neutrino, a ghostlike particle that can easily zip through Earth without interacting with anything. Neutrinos were formed in huge quantities in the maelstrom of the early universe and continue to arise in nuclear reactions and radioactivity. For a long time, physicists thought the neutrino had no mass, and so couldn’t add to the dark matter, but many now think it does. Although each neutrino would weigh something like 500 million times less than a proton, so many of them populate the universe that they could make up a significant chunk of the dark matter. Yet when cosmologists working with computers simulate neutrino-based universes, they seem to bear little resemblance to the observable cosmos.

       At least physicists know that neutrinos exist. That’s not the case for a group of theoretical particles known as WIMPs, another playful acronym, this one standing for Weakly Interacting Massive Particles. These ghostly particles have predicted masses ten or more times the mass of a proton and should be numerous, so they could add a lot of dark matter. Cosmologists like neutrinos, WIMPs, and other exotic particles because they could have been the gravitational seeds around which ordinary matter collected to form galaxies.
 

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