"How do we measure the age of the Universe?" asks Simon Whitehead.

A hundred years ago this wouldn't even have been considered a valid question, because we didn't think the Universe had a beginning at all. Even Einstein thought that space was eternal and unchanging.

This is the tale of how we discovered that the Universe had a beginning, and why calculating its age has been one of the greatest challenges in modern astronomy.

We also uncover the mysterious dark energy that pervades the cosmos and discover why it's been putting a scientific spanner in the works.

Helping to unravel today's question are physicists Andrew Pontzen, Jo Dunkley and Jim Al-Khalili.

Presenters: Hannah Fry, Adam Rutherford
Producer: Michelle Martin.