When you look up at night, you see myriads of stars spread across the sky. When astronomers look into the deepest reaches of the universe with powerful telescopes, they see myriads of galaxies, organized into large clusters and other structures. This might lead you to believe that the universe is composed mainly of galaxies, stars, gas and dust – things that you can see. However, most astronomers believe that visible matter makes up only a small fraction of the mass of the universe. The majority of the universe is made of stuff we can’t see – so-called dark matter.
What is dark matter? Simply put, dark matter cannot be seen by astronomers with telescopes. It doesn’t emit or reflect enough light to detect, so it’s not bright, like a star. Atoms, molecules and subatomic particles are dark matter. You and I are dark matter. Everything on Earth is dark matter. Planets, brown dwarf stars and black holes are dark matter. Basically, dark matter cannot be seen ̶ scientists can only estimate where it is based on gravitational effects on what they can see.
Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun. The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles).
MACHOs are made of ‘ordinary’ matter, which is called baryonic matter. They are non-luminous objects that make up the halos around galaxies. Machos are thought to be primarily brown dwarf stars and black holes. Like many astronomical objects, their existence had been predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation. Black holes were predicted by Albert Einstein’s General Theory of Relativity.
WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs. In their efforts to find the missing 90% of the universe, particle physicists theorize the existence of tiny non-baryonic particles that are different from what we call “ordinary” matter. Smaller than atoms, Weakly Interactive Massive Particles are thought to have mass, but usually interact with baryonic matter gravitationally – they pass right through ordinary matter. Since each WIMP has only a small amount of mass, there needs to be a large number of them to make up the bulk of the missing matter. That means that millions of WIMPs are passing through ordinary matter – the Earth and you and me – every few seconds. Although some people claim that WIMPs were proposed only because they provide a “quick fix” to the missing matter problem, most physicists believe that WIMPs do exist. According to Walter Stockwell, astronomers also concede that at least some of the missing matter must be WIMPs. The problem with searching for WIMPs is that they rarely interact with ordinary matter, which makes them difficult to detect.
A dark matter discovery could possibly affect our view of our place in the universe. If scientists prove that non-baryonic matter does exist, it would mean that our world and the people in it are made of something which comprises an insignificant portion of the physical universe. A discovery of this nature, however, probably will not affect our day-to-day process of living. Perhaps the only thing a dark matter discovery will give us is some perspective.