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Why dark matter may not be so dark after all (+video)

A duo of physicists at Vanderbilt University have proposed a straightforward model that could explain dark matter in terms of known phenomena. 

Lesson by James Gillies, animation by TED-Ed
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Physics is the study of matter and energy, so a fundamental question of physics is how much of it there is. But every time physicists try to calculate the amount needed to hold everything together, they always seem to come up short.

And not just by a little bit, either.

Based on observations of how galaxies and clusters of galaxies rotate, how light seems to bend around certain galactic clusters as though they were more massive than they appear, and how the Big Bang's 'afterglow' is distributed throughout the cosmos, scientists have concluded that almost 85 percent of the universe remains unaccounted for.

The most straightforward explanation, say physicists, is that there exists something that makes itself known only indirectly, by tugging on stars and light with its gravitational pull, but that otherwise resists observation. 

They call this something "dark matter." According to recent observations, it makes up 26.8 percent of the universe. Another mysterious substance, called "dark energy," thought to make up 68.3 percent of the universe, is believed to be responsible for accelerating the rate at which the universe expands.

The rest is made up of what physicists call "ordinary matter," that is, all the stars, planets, rocks, animals, people, heat, light, radio waves, cosmic rays and everything else that normal people imagine when they hear the word "everything." All that accounts for just 4.9 percent.

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