We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

Because the galaxy is rotating, the stars in it tend to want to fly off outward. The only thing holding them together is the force of gravity from all the mass present in the galaxy (gravity pulls things with mass together). The faster the galaxy spins, the more mass you need to hold all the stars in. Conversely, knowing the mass of the galaxy means you can predict how fast the galaxy can spin.

Astronomers at first tried to guess the mass of galaxies by counting the number of stars in them. But when they used this number to compute how fast galaxies should be spinning, something didn’t match up. Measurements showed that the galaxies were spinning faster than was predicted by how many stars they contained. In other words, the stars should be flying off the edges of the galaxies, just like the Ping-Pong balls in the merry-go-round.

Gravity If two things have mass, they will feel an attractive force toward each other.

Electromagnetism This is the force that two particles feel if they have an electric charge. It can be attractive or repelling depending on whether the charges are different or the same.

A simple and concrete idea is that dark matter is made of a new kind of particle that uses a new kind of force to interact very, very weakly with normal matter.

This driving force that’s making the universe bigger at an increasing rate is what physicists call dark energy.