Although there’s no way to be really sure, scientists have found a clever way to figure out the age of the universe, or at least to estimate how long it’s been around in its present form.
Many scientists believe that the universe began with a tremendous explosion that sent all matter flying out into space from some central point. The idea that the universe began with a big explosion is called the big bang theory.
One of the reasons scientists believe the big bang theory is that the universe seems to be expanding. All stars and galaxies are flying away from each other, as if they had been shot into space by that first explosion.
Scientists can measure the speed of stars in our part of the universe, then measure the speed of stars in other, distant parts. Since light from very distant stars takes billions of years to reach us, then looking at the way stars are moving in distant parts of the universe is like looking at how those stars were moving billions of years ago.
By comparing the speed of stars now with the speed of stars long ago, scientists can figure out when the stars began to move through space. And they’ve learned that the universe is at least ten billion years old!