“The universe is huge” is true in the same way “the ocean is wet” is true: it’s correct, but it doesn’t prepare you for the experience. Cosmic distances are so extreme that the numbers stop behaving like normal numbers and start feeling like poetry. Still, with a few carefully chosen yardsticks—light-years, horizons, and the time it takes light to travel—we can build an intuition for what astronomers mean when they talk about the size of the universe.

When people ask “How big is the universe?”, they may mean at least three different things: the size of the observable universe (what we can in principle see), the size of the universe as a whole (which may extend far beyond what’s observable), and the size of the universe through time (because the universe expands). Let’s take those one at a time, without needing a degree in cosmology.

The first ruler: what does ‘distance’ mean in space?

On Earth, distance is a straight line you could—at least in theory—walk. In space, distance is still a straight line, but it’s so large that kilometers quickly become useless. Astronomers use the light-year: the distance light travels in one year. It’s not a time unit dressed up as distance; it’s a genuine measuring stick. If you could hop on a laser beam and ride it for a year, you’d cover one light-year.