Galaxies — those vast collections of stars that populate our universe — are all over the place. But how many galaxies are there in the universe? Counting them seems like an impossible task. Sheer numbers are one problem — once the count gets into the billions, it takes a while to do the addition. Another problem is the limitation of our instruments. To get the best view, a telescope needs to have a large aperture (the diameter of the main mirror or lens) and be located above the atmosphere to avoid distortion from Earth's air.
Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field (XDF), an image made by combining 10 years of photographs from the Hubble Space Telescope. The telescope watched a small patch of sky in repeat visits for a total of 50 days, according to NASA. If you held your thumb at arm's length to cover the moon, the XDF area would be about the size of the head of a pin. By collecting faint light over many hours of observation, the XDF revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. So if that single small spot contains thousands, imagine how many more galaxies could be found in other spots.
While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies, said Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, Maryland. When the James Webb Space Telescope launches in 2020, the observatory is expected to reveal even more information about early galaxies in the universe.