Much of what happens in the universe is not easily observable.
The observable universe, also known as the Hubble volume, is the region of space that we can theoretically observe, small enough that light from more distant regions has had enough time to reach Earth since the Big Bang. This region of space is approximately 92.94 billion light-years in diameter, centered on planet Earth. Each different portion of space has its own visible universe, some overlapping, some not.
The part of space that humans can see is known as the observable universe.
There are several misconceptions about the concept of an observable universe. The first is that its size is equal to its age in years multiplied by the speed at which light travels in one year, about 15 billion light-years. This figure would be true if it were flat, but thanks to Einstein we know that the universe is very curved at cosmological distances by virtue of its expansion.
Space telescopes can be used to determine the age of the universe.
The other misconception is that what is observable is all there is. This is also false; indicates only the region of space that we can theoretically observe. The real universe could be much larger, or even smaller. There is a possibility that the distant galaxies we see are actually light from closer galaxies, which have circumnavigated the universe more than once. Since the light would have radically different ages, it would be difficult, if not impossible, to say that they were from the same galaxy. But, in all probability, the universe itself is much larger than what can be observed.
Our galaxy, the Milky Way, is believed to have up to 400 billion stars.
The terms “universe” and “observable universe” are sometimes used interchangeably by cosmologists because, by definition, regions outside of what we can see are causally disconnected from us. We cannot observe, let alone influence, these regions, and the reverse also applies. In addition to being spatially gigantic, our entire universe is likely just one in a larger set of parallel universes known as the multiverse.