It’s a great question and also a complicated one.
When you study the mysteries and philosophy, you discover that the universe is made of numbers.
Modern physics agrees with this concept and it is easily demonstrable by studying atomic particles and the fundamental laws of the universe.
The sages of the ancient world taught this as well in the “secret doctrine” as did great philosophers such as Hippocrates and Pythagoras.
It goes essentially like this:
From nothing comes the 1, then 1 divides itself into 2, the 2 then produces a 3rd element.
(For example the Atom is made of 3 parts. the electron, the neutron and the proton.)
This all important “3”. (The trinity) gives birth to the physical universe.
(Thus the 3 gives birth to 4.)
This holy ‘3’ (the un-manifest trinity, the creative power of G-d.) plus ‘4’ (the physical realization of that unseen creative power) equals 7.
7 is special because it represents a kind of completion of the first cycle creation. (e.g. G-d created the world in 6 days and rested on the 7th.)
For more on this topic I recommend this article: