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What is Dark Matter Anyway?
Dark Matter is a purely theoretical type of matter that accounts for more than 80% of the matter in our universe. The original theory came about in 1932 by a Dutch astronomer named Jan Oort, who used dark matter as a way to explain movements of stars in the Milky Way. While studying these movements, he realized that the mass of what was seen did not line up with the actual movements of stars. Therefore he inferred that the mass of the galaxy must contain other types of imperceptible matter whose gravity is strong enough to change the movement of stars and galaxies. Now, dark matter is generally accepted as a fact in most scientific circles, because it is the best solution to the 'missing matter' in galaxy rotation curves (a plot of star velocity versus radial distance from the center of the galaxy).
So, What Could Dark Matter Be?
There are a few theories as to what dark matter actually is, but the most widely accepted is that it is mosty comprised of WIMPS (Weakly Interacting Massive Particles) that only interact through gravity and the weak nuclear force. There are also theories of a parallel universe made of dark matter, where the only thing that can leech through the barrier is gravity. Dark matter is theorized to be complex, just as our baryonic matter is. There is probably not one singular kind of dark matter, but instead many different types that work in different ways. One theory that uses this idea is that dark matter particles interact with each other, forming the possibilty for a 'dark force' only felt by dark matter. Whatever the case may be, if dark matter does exist most of it must comprise of WIMPs. We know this because if there were a perfectly alternative dark universe, the dark energy would start forming their own galaxies because of dark photon emissions. However, the dark matter we observe forms spherical clouds of matter instead of discs.
Will We Ever Know For Sure If Dark Matter Exists?
With the startup of the LHC at full power, some interactions between WIMPs and baryonic matter could be observed, leading to proof of dark matter, which would be one of the greatest discoveries of the century. Until then, it remains a theory, albeit a widely accepted one.