What is "dark matter"?
My teacher was talking about dark matter in class today, and I still don't get what he was talking about. What is dark matter, exactly?
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It's not surprising that you don't quite understand what dark matter is--because your teacher doesn't either. Neither do scientists. Nobody knows what dark matter is, exactly, or even whether or not it really exists--but scientists have good reason to believe that it does.
Scientists measuring the mass of galaxies--or how much matter is packed into each galaxy--have found some discrepancies in their measurements. Their measurements of the mass in galaxies--based on the way gravity interacts with objects and matter in those galaxies, plus measurements of light, rays and waves, and other forms of energy emitted from them--suggest that there's a whole lot of matter out there we can't detect. The visible matter in the universe--including stars, planets, asteroids, comets, neutron stars, black holes, dust and gas, subatomic particles, and so on--accounts for only about 10% to 1%--depending on which scientist you talk to--of the matter that's actually there.
As for the other 90% to 99% of matter in the universe, scientists have no idea what it is. They speculate it could be a strange form of subatomic particle that doesn't interact with other matter the way others do; or maybe super-massive black holes. Scientists have terms for these two possible sources of dark matter; they're called WIMPs (weak little subatomic particles that don't interact with anything, which is why we can't detect them) or MACHOs (big, beefy super-massive black holes and other huge, dense stellar objects we don't know about because they're too far away to be sensed).
And dark matter is only part of the story. Scientists theorize that dark energy makes up a big part of what's missing in the observable universe--bigger even than dark matter. The universe is expanding--galaxies are racing away from each other--and scientists have noticed that the expansion isn't slowing down, but accelerating. Why is it speeding up? We don't know, but something is pushing the acceleration faster than can be accounted for by gravity and other forms of energy in the universe. That's dark energy.
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