We know of 3 dimensions. But what if there are more? And if so why can’t we see them?
We can define the universe by about 20 numbers, such as the strength of the gravitational force, the weight of an atom, the cosmological constant etc.. But how can we get these numbers unified in one simple equation that defines everything? Could it be that we need to have additional dimensions to unite these known numbers?
Yes, according to scientists but we need at least 10 dimensions.
How come then can we not see these extra dimensions? The reasoning is that these extra Ds are actually infinitesemal tiny, very tiny. they actually exist in the particles called ‘quarks’ that form the proton and neutrons of an atom. These extra Ds can only be appreciated inside these very very tiny quarks and if you see them, the extra Ds actually look like vibrating strings of electromagnetism.
This is called the String Theory of Matter, which might actually represent the Holy Grail of Physics: The Unifying Force of Nature. 
The Global Soul 