What is the Universe made from? - Current Big Questions in Cosmology

When astrophysicist Vera Ruban looked at Galaxies, she noticed a curious problem. She expected that the outer parts of a galaxy would move slower than the inner parts. But she found that this is not the case. The rotation curves of galaxies (a graph of the radius of a galaxy versus rotational speed) is flat, meaning that the outer parts move at the same speed as the inner parts. Large amounts of mass would account for the unexpected speed, but we don't see the mass that should be there.
To aid your understanding of this, think of how planets revolve around the Sun in our solar system. Mercury (the closest planet to the Sun) zips around the Sun in 88 days, but it takes the furthest planet, Pluto, 248 years to orbit the Sun. If there were a solid sphere of mass between the Sun and Pluto, than Pluto's orbital period would be the same as Mercury's. No one is suggesting that galaxies are actually solid spheres of matter, but there must be more mass in these galaxies then we can see. Because we can't see it, the mass is called dark matter.
Dark matter may account for up to 90% of the Universe's total mass. Bernard Sadoulet, who leads a search for dark matter at theCenter for Particle Astrophysics has stated that "Not only can we not see what most of the Universe is made of, we aren't even made of what most of the Universe is made of!" What did he mean by this? Scientists tend to categorize everything and matter is no different. The matter you are familiar with, matter composed of neutrons and protons, is called baryonic matter. Non-baryonic matter also exists, but is generally difficult to detect. Professor Sadoulet's experiment is looking for exotic, non-baryonic particles called WIMPs. WIMP stands for Weakly Interacting Massive Particle. There is a great deal of theoretical work which suggests that WIMPs exist and probably account for a large fraction of dark matter.
If you don't believe that WIMPs exist, you aren't alone. But some sort of exotic non-baryonic dark matter is required for omega = 1. Big bang nucleosynthesis limits the total number of baryons to be a fraction of the Universes total mass. And since there are compelling reasons to believe big bang nucleosythesis, and also that omega = 1, one is led to the conclusion that there must be exotic non-baryonic dark matter. Note the use of the word exotic. Neutrinos are another type of non-baryonic particle, but are not considered exotic. Neutrinos do exist, in huge numbers, but all known neutrinos have zero mass. The search continues for neutrinos with mass, but a massive neutrino is unlikely to completely account for the flat nature of galactic rotation curves. Hence, an exotic class of non-baryonic dark matter particle must exist if WIMPs do not.
There are several candidates for baryonic dark matter. MACHOs (Massive Compact Halo Objects) are objects about the size of Jupiter. Jupiter is quite massive, but like all planets, does not emit any light of its own; it only reflects sunlight. Although we can see Jupiter quite well from Earth, chances are that someone looking at our solar system from any far distance would not be able to see Jupiter. So, it is reasonable to assume that there are Jupiter-sized objects in other solar systems that we cannot see. By a technique known as micro-lensing, several MACHOs have already been found. VMOs (Very Massive Objects) are about 100 times more massive than our Sun, which makes them very heavy indeed. They are likely to be found in the form of black holes. By the way, in case you're wondering whether the existence of many Earth-sized objects might account for all the dark matter, bear in mind that Jupiter is roughly 300 times more massive than Earth. Thus you would need so many Earth-sized objects that the galaxy would be littered with them