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Why are scientists so interested in dark matter?

Probably because there's more "dark" matter than there is "regular" matter!

We call it "dark" because we can't see it, nor can we clearly pinpoint its location by any other means; but our best theories of cosmology and astrophysics tell us that about 80% of the matter in the universe (or 25% of the total energy in the universe) is comprised of this stuff we call "dark matter". It doesn't seem to interact with most of the forces we know; in particular it's not susceptible to electromagnetism, and we don't know if it's susceptible to the nuclear forces. But it definitely is affected by---and more importantly, affects---gravity, and that's how we know it must be there.

Without dark matter, we would be unable to explain why galaxies are stable. Ordinary matter clumps too much, creating inhomogeneities in the distribution of mass that lead to tighter and tighter clumps of matter---you know many of these clumps as "stars" and "planets". But based on how fast galaxies are spinning, the orbits of those clumps of matter would become unstable, without some other source of mass to hold them in place---that other source is dark matter. Without it, we probably couldn't be here.

Another reason why scientists are so interested in it is precisely because we don't know what it is. Scientists want to find out! That's where science is most exciting---the things we don't yet understand. Dark matter wasn't predicted by our theories of particle physics, yet it keeps showing up in our theory of gravity; so what is it? Where does it come from? Whoever finally figures it out is going to be a shoo-in for the Nobel Prize.

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