If you keep travelling in an imaginary faster-than-light spaceship, would you ever arrive at some boundary, unable to go any further? And if so, what lies beyond? It's all very hard to imagine. Then again, an infinite Universe is just as difficult to wrap your head around. After all, there must be something that space is expanding into, right?
Let's start with a related but easier-to-grasp concept: our Universe has an apparent edge, called the cosmological horizon. The light emitted right after the Big Bang has been travelling for 13.8 billion years through space. This means we can only see the Universe up to a current distance corresponding to a light-travel time of 13.8 billion years. Thanks to the expansion of space, this so-called co-moving distance is approximately 45 billion lightyears, and anything beyond this limit is unobservable to us because not enough time has elapsed since the birth of the Universe for light from these remote regions to reach our telescopes.
But, just like the familiar horizon seen by sailors on the ocean, this cosmological horizon is not some real, physical boundary. And as the ocean stretches beyond the sailor's horizon, so too does space stretch beyond our observable Universe. There's no reason why there can't be galaxies at these extremely large distances; they're just invisible to us, no matter how powerful our telescopes are.
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