Lets start this article with a fact that might blow your mind:
“Our Universe is just 13.8 billion years old but it has a radius of 46.5 billion light years.”
What does this mean? Our Universe was created in a Big Bang about 13.8 billion years ago and has been expanding ever since then. The idea of an expanding universe dates back to 1917, when Albert Einstein noted that the equations in his general theory of relativity didn’t work well for a static universe. So, he threw in a hypothetical repulsive force (dark energy) that would fix this problem.
It was not until 1920 that astronomer Edwin Hubble discovered that the universe was actually expanding. This was a turning point in astronomy and led to the concept of the Big Bang theory.
The Hubble Space Telescope that was launched in 1990 has very accurately measured the rate of expansion of our universe. According to Hubble, the outer edges of the observable universe are expanding faster than light! This is the reason why the observable universe has a radius of 46.5 billion light years and not just 13.8 billion light years!
Today, we have enough evidence for the expansion of our universe. But if our universe is expanding then what is it expanding into? Before answering this, let us first get a brief idea about the shape of our universe.
Our universe as we know, can either be finite or infinite. Also, based on its density, it can have one of the following curvatures: no curvature (flat), positive curvature (open) or negative curvature (closed). These factors result into a combination of different shapes for our universe such as cylinder, sphere, toroid, etc. According to our best measurements, our universe appears to be flat and infinite (never ending).
Keeping that in mind, now lets get back to the main question. The simple answer to this question is that the universe isn’t expanding into anything. It is just that the distance between galaxies is increasing continuously. The galaxies are relatively still but the space between them is moving. Hence, we can say that the galaxies are not moving through space but they are moving in space.
To understand this, consider the numbers 1, 2, 3, …………….up to infinity. Now multiply each of them by 2. You get 2, 4, 6, ………….up to infinity. Here, after the multiplication, the distance between the adjacent numbers has increased but the total extent of numbers remains the same!
Exactly the same thing happens with our universe. The space between the galaxies is increasing. However, since the universe was infinitely big to begin with and will remain infinite, it really doesn’t expand into anything. The illustration below might help you to get a better understanding.
Hope this article answers your questions. If you have any doubts/queries feel free to ask them in the comments.