When you look up past the stars of our Milky Way and out at the galaxies beyond, it might surprise you to learn that most of what we see isn’t most of what’s actually there. Sure, in our Solar System, 99.8% of the mass is in our Sun, and astronomy has taught us a tremendous amount about how stars work. So you might think that if you measure all the starlight — of all different types and wavelengths — coming from each individual galaxy we observe, we can figure out how much mass is in there.
On the other hand, we know how the laws of gravitation work, and how the motions of gravitationally bound objects depend wholly on the total mass of the system and how that mass is distributed. So we can look both at individual galaxies and how the stars within them orbit, as well as how entire galaxies move within giant galactic clusters. When we make all of these measurements, we find a shocking fact: the measurement of mass from light and the measurement of mass from gravitation are off from one another by a factor of 50.
Now, we’ve discovered lots of other types of matter in the Universe besides stars, including:
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stellar remnants like white dwarfs, neutron stars and black holes,
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asteroids, planets and other objects with masses too low (like brown dwarfs) to become stars,
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neutral gas both within galaxies and in the space between them,
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light-blocking dust and nebulous regions,
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and ionized plasma, found mostly in the intergalactic medium.
All of these forms of normal matter — or matter originally made of the same things we are: protons, neutrons and electrons — do in fact contribute to what’s there, with gas and plasma in particular contributing more than even stars to. But even that only gets us up to about 15-to-17% of the total amount of matter we need to explain gravitation. For the rest of it, we need a new form of matter that isn’t just different from protons, neutrons and electrons, but that doesn’t match up with any of the known particles in the Standard Model. We need some type of dark matter.
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