This has a pretty interesting answer I think!

It is because of our brains and language.

You see, colors do not actually exist. What we perceive as colors are just different wavelengths of light.

We don't have to go into what wavelengths are, just see them as a quality light has that can differ.

Eyes evolved to see a certain spectrum of light. That means a certain slice of all wavelengths light can have, the rest is invisible to us; like radio waves or x-rays!

The human brain evolved to divide this visible light so we can make out subtle differences, most likely like seeing a tiger in the grass, this is the sensation of color.

So actually, rainbows are just a gradient of wavelengths, but we perceive that as individual colors.

But the weird thing is, just how individual they are perceived to be seems to be closely linked to language.

Some languages only have three words for all colors: red, black and white. What seems like blue to you, a native speaker of that language might say is black. What you say is purple they might say is red.

Some languages have four colors and so on.

English, not counting hues, generally has 6 main colors. Red, orange, yellow, green, blue, (indigo), violet. Does that ring a bell?

That's the colors the rainbow is said to have in English (and many other languages). Isn't it strange and kind of awesome?

You can for sure have infinite numbers plus one, or divided by two.

Those kind of numbers are called transfinite numbers (ω, ℵₒ, m)

They are not finite, just not absolutely infinite (∞).

But a transfinite number t + 1 is, well, (t + 1) and t / 2 is (t / 2)

Which is not the case for ∞ + 1 or ∞ / 2 which both are still ∞

Maybe I shouldn't go into transfinite numbers in eli5...

Not just a nitpick but a qualified correction! I have updated my reply

Simply because the universe, to the best of our knowledge, has no center.

We cannot say for sure but if our physics is right, the universe is very likely unbound and infinite. There is no edge and no end.

Then there is the thing we call the observable universe. It is what it sounds like, the universe we can observe.

Because light takes time to reach us, we are looking back in time the further away we are looking in space.

We can look so far back, the universe was a opaque gas cloud everywhere, and very little light from that time escaped that gas. Eventually we hit a theoretical stop, the birth of the universe, and we cannot look further back than that.

Where that edge is lies our cosmic horizon.

So our observable universe has a size, and that is 13.8 billion light years (the age of the universe) + the expansion of space, so 46.5 billion light years in every direction from here.

Actually, not here, but from wherever you are observing.

You are in the most present you can be, which also means you are in the most center position you can observe the universe, which means you are always in the center of the observable universe.

You are literally the center of the universe and with science to back it up.

But unfortunately that is not so special, since that is true for everyone and every point that exists.

But for the whole infinite universe, if it is truly infinite, there is no center.