"No one knows if the universe is infinitely large, or even if ours is the only universe there is."
Read the document from Harvard website.
Throughout history, humans have used a variety of techniques and methods to help them answer the questions 'How far?' and 'How big?' Generations of explorers have looked deeper and deeper into the vast expanse of the universe. And the journey continues today, as new methods are used, and new discoveries are made.It was knowing this fundamental distance from the Earth to the Sun that helped us find the true scale of the entire Solar system for the first time.When we leave the solar system, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to travel across it.The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to stars that are very far away.Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies we discover. We see them not as they are today, but as they looked long before there was any life on Earth.Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness so we can figure out how far they are by measuring how bright they appear to us, and therefore how far away it is to their home galaxy.
Adapted from NASA website