It’s not a book about the known facts regarding the nature of the universe so much as a book about the process by which those facts were determined and became accepted.
Q: Well, then, what do astronomers mean by “dark matter”?
Panek: A mysterious substance that comprises about 23 percent of the universe.
Q: And dark energy?
Panek: Something even more mysterious that comprises about 73 percent of the universe.
Q: Okay, 73 and 23 add up to 96 percent, which does leave a four percent universe. But if we don’t know what dark matter and dark energy are, how do we even know they’re there?
Panek: In the 1970s, astronomers observed that the motions of galaxies, including our own Milky Way, seem to be violating the universal law of gravitation. They’re spinning way too fast to survive more than a single rotation, yet we know that our galaxy has gone through dozens of rotations in its billions of years of life. Galaxies are living fast but not dying young—a fact that makes sense only if we say that there’s more matter out there, gravitationally holding galaxies and even clusters of galaxies together, than we can see. Astronomers call this substance dark matter.
Q: And the mysterious dark energy?
Panek: In the 1990s, two independent teams of astronomers set out to discover the fate of the universe. They knew the universe was born in a big bang and has been expanding ever since. Now they wanted to know how much the mutual gravitation among all this matter—dark or otherwise—was affecting the expansion of the universe. Enough to slow it down so that the universe would eventually grind to a halt, then collapse on itself? Or just enough that the expansion would grind to a halt and stay there? In 1998 the two teams came to the same conclusion: the expansion of the universe isn’t slowing down at all. In fact, it’s speeding up. And whatever force is counteracting gravity is what they call dark energy.