The concept of the expansion of the universe can be quite complex to grasp fully, but I'll do my best to explain it in simple terms.
When we say that the universe is expanding, we mean that the space between galaxies and other celestial objects is getting larger over time. However, it's important to note that it's not the objects within the universe that are moving away from each other, but rather the space itself that is stretching. Think of it like the surface of a balloon being inflated—points on the balloon move farther apart as the balloon expands, even though the points themselves aren't actively moving.
The place captured by the universe, so to speak, is the entirety of space and all the matter and energy within it. There isn't a specific location where the expansion is happening because it's happening everywhere simultaneously. Every point in the universe experiences this expansion, leading to an overall increase in the distances between objects.
As for where the expansion comes from, the leading explanation is a theory called the Big Bang. According to this theory, the universe began as an extremely hot and dense point, often referred to as a singularity. Approximately 13.8 billion years ago, a rapid expansion occurred, resulting in the formation of the universe as we know it today. Since then, the expansion has been ongoing, causing the universe to become larger and less dense over time.
It's worth noting that while the concept of the universe's expansion is supported by a wealth of observational evidence, the exact mechanisms driving this expansion and its ultimate fate are still active areas of research and study in cosmology.