How did the universe fit into the size of an atom?

Since it is not an observable phenomenon, we are unable to really understand how the initial singularity, which is theorized to have existed before the universe began, could have been infinitely dense. The most similar thing would be the singularity of a black hole, which is also said to have infinite density, though some infinities are bigger than others.

Using this logic, the singularity of a black hole would be like the number of numbers between 1 and 2, and the singularity of the big bang would be like the number of numbers between 1 and infinity. For instance, there are an infinite number of numbers between 1 and 2, you just keep adding decimal points. Now there are an infinite number of numbers between 1 and 3 as well. The second infinity would be larger than the first infinity.

The idea you're referring to is called the "Big Bang Theory." This theory states that approximately 13.8 billion years ago, the universe experienced a sudden and tremendous expansion from a state of extreme heat and density. In the beginning, the universe was incredibly small, dense, and hot—possibly even smaller than an atom. As a result of this expansion, space itself stretched and cooled, eventually resulting in the formation of galaxies, stars, and planets over billions of years.