Ask Ethan: Why doesn’t radiation dominate the Universe?
<p>Today, our Universe is filled with stars and galaxies, and is not only expanding, but the Universe’s expansion is accelerating. If we were to break up the Universe into the different types of energy that compose it, we’d find that it was dominated by dark energy, which makes up 68% of the Universe’s energy density. Next would be dark matter, as it composes some 27% of the Universe’s energy density, followed by normal matter (protons, neutrons, and electrons), that makes up about 4.9% of all that’s out there. The other 0.1%? That’s made of things like neutrinos and photons, where all photons and the fastest-moving neutrinos both behave as forms of radiation.</p>
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