This idea stems from General Relativity, which shows that space and time are not fixed but dynamic and interwoven. Two key discoveries in the early 20th century solidified this understanding. First, Vesto Slipher observed that light from many nebulae was redshifted, indicating they were moving away. Second, Edwin Hubble measured distances to these galaxies and found that the farther they were, the faster they receded. This correlation, now known as Hubble’s Law, confirmed that the Universe is expanding.
Scientists often use analogies to explain this phenomenon. The “balloon analogy” imagines galaxies as coins on a balloon’s surface, moving apart as the balloon inflates. Another analogy is a loaf of raisin bread dough, where the raisins (galaxies) move apart as the dough (space) expands. However, these analogies fall short in some respects. Unlike the dough or balloon, the Universe doesn’t expand into anything; it’s all there is.
Observations suggest the observable Universe is only a fraction of a potentially infinite cosmos. While light from unseen regions will eventually reach us, expanding spacetime itself ensures galaxies continue moving farther apart. The theory of cosmic inflation suggests that our Universe is one “bubble” in a vast multiverse, though these regions remain isolated from one another.