Relative to what?
(Would we not need a measuring device external to our universe?)
Good question.
When you start reading about cosmology, a lot of it borders on the fantastical but the facts and number are there.
Check out some popular books on quantum theory.
Read about how light behaves. Sometimes it behaves like its a wave - sometimes it behaves as though its made of discrete particles.
Einstein said matter and energy are convertible and matter warps space-time. He also said that the speed of light is constant and that you can’t go faster than light. (Recent studies indicate this may not be the case)
He also said that if you go fast enough, a clock slows down and a person traveling at infinite speed ages slower than people who are moving slowly - or vice versa - I can’t remember.