The universe was born in the Big Bang — a single moment in which matter, energy, and even space and time sprang into existence. And in recent decades, scientists have been zeroing in on just when the Big Bang took place. Their conclusion: about 13.8 billion years ago.
Several lines of evidence support that age.
One is the rate at which the universe is expanding as a result of the Big Bang — and how that rate has changed over time. If you know how fast things are moving apart, then you can “rewind” the clock and figure out when they started moving apart. Measuring the expansion rate isn’t easy, though. And astronomers still aren’t in full agreement on what the rate really is.
Another line of evidence is the ages of stars. The oldest stars were born at least 13 billion years ago. So the universe must be older than that.
And yet another bit of evidence is the chemistry of the universe. The Big Bang created hydrogen and helium, plus a tiny smattering of a couple of other elements. Everything else was forged in the hearts of stars. Astronomers know how quickly that happens. So they can calculate how long it should take to build up the amounts of oxygen, carbon, and other elements we see in the present-day universe. And the result lines up well with the other lines of evidence.
Together, these and other bits of evidence tell us that the universe is about 13.8 billion years old.