The most widely accepted explanation explaining the universe’s beginnings is the Big Bang theory.

The universe is expanding, according to data published by Edwin Hubble in 1920. Galaxies are getting farther apart with the passage of time. For instance, draw some points on the balloon; when it is blown, the points begin to drift apart. In the same manner, the universe is thought to be expanding because of the rising distance between galaxies. However, since the balloon’s marked points grow as well, this is only partially accurate. Scientists believe that even while there is a rising distance between galaxies, these observations do not support the idea that the galaxies are expanding.
The Big Bang theory states that:

All of the substance that makes up the universe was once contained in a single, minuscule ball with an infinitely small volume, infinite density, and limitless temperature.

13.7 billion years ago from the present Big Bang occurred. The tiny ball exploded violently and it is believed that the expansion continues even to the present day. The rapid expansion took place within fractions of a second after the bang and slowed down.

After around 300,000 years, the universe became transparent due to a drop in temperature that gave origin to atomic matter.

So according to the Big Bang theory, the universe began from a small point, and it  has been expanding and changing ever since then, creating different matters of the universe.