How is it possible that scientists estimate that the universe is 13.7 billion years old, but more than 90 billion light-years across?

Galaxies galore.

NASA/ESA/S. Beckwith (STScI)/HUDF Team

The total distance in light-years as measured today is therefore greater than the actual time of travel as measured by a stopwatch multiplied by the speed of light.

Using this logic and some equations, one can show that, given the universe’s age of 13.7 billion years, the most distant object we could possibly see in the universe today is about 47 billion light-years away.