This post is the fourth in a series that accompanies the publication of my book ‘Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos’ (Scientific American/FSG).

Ten years ago the universe was in trouble.

Or rather, our puny human theories about the nature of all the stars and galaxies in the universe were in trouble. At this time we had managed to discover a number of remarkable things about our cosmic surroundings. We had detected minute non-uniformities in the remains of the primordial radiation field known as the cosmic microwave background, and we had ever more convincing evidence that normal matter – the stuff we are made of – amounts to barely a fifth of the total raw mass of the universe. The rest is dark matter, a shadowy stuff that seldom interacts particle-to-particle but is massive enough that its gravity dictates much of the motion of stars in galaxies and the colossal structures of galaxy clusters and superclusters.

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