How many galaxies are in the universe?

The Milky Way is just a speck in a universe filled with an untold number of galaxies. But if we had to take an educated guess, how many galaxies are in the universe?

That sounds like a simple question, but it's anything but. The first problem is that even with our most powerful telescopes, we can see only a tiny fraction of the universe.

"The observable universe is only that part of the universe from which the light has had time to reach us," astrophysicist Kai Noeske, now outreach officer at the European Space Agency, told Live Science.

The universe is 13.8 billion years old, but the observable universe stretches more than 13.8 light-years in every direction. That's because the universe is expanding and light got a head start early on, when the universe was smaller.

"Now, the total size in each direction is about 46 billion light-years," Noeske said.

That's much smaller than even our smallest estimates of the entire universe. "We see at most 3% of the universe," Pamela Gay, a senior scientist at the Planetary Science Institute, told Live Science.

Related: Why are galaxies different shapes?