The Milky Way is just one dot in a universe filled with countless galaxies. But if we had to make an educated guess, how many galaxies are there in the universe?

This seems like a simple question, but it is not. The first problem is that even with our most powerful telescopes we can only see a very small part of the universe.

“The observable universe is just the part of the universe where light reaches us,” astrophysicist Kai Noeske, now a public affairs officer at the European Space Agency, told LiveScience.

The universe is 13.8 billion years old, but the visible universe extends 13.8 light-years in each direction. This is because the universe is expanding and light was favored when the universe was smaller.

“The total size in all directions is now about 46 billion light-years,” Noeske said.

That’s much smaller than our smallest estimates of the entire universe. “We’re seeing, at most, 3% of the universe,” Pamela Gay, a senior scientist at the Planetary Science Institute, told LiveScience.