The math of the law of gravity and astrophysics work. They work well enough to find exoplanets, other galaxies, to accurately calculate orbits, trajectories and speeds and well enough to figure out other fantastic and amazing things.
The problem is that in order for the math to work we have to assume that there is more matter in the universe than we can actually see. We know, or are at least pretty sure, that the dark matter is there, because without it all of those accurate calculations and discoveries would be wrong. But what exactly is this "dark matter" that we can't observe?
This short video from Scientific American looks at some of the leading candidates and theories.
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