Observation -> Our universe is expanding at a certain rate.
To let the universe expand there has to be a driving force.
That force must be bigger than the sum of gravity which tries to contract it.
But not bigger than the speed of expansion would allow it to be.
There is a certain amount of detectable matter in our universe.
That amount of detectable matter/energy isn’t enough to explain the phenomenon.
The phenomenon is still there.
Science requires scientist to think harder.
Scientist comes up with a great solution.
We need more mass.
Because of quantum fluctuations, matter constantly ‘pops’ in and out of excistence.
So on a quantum level empty space actually has energy.
Energy is mass.
Empty space has mass.
Voila, we have so called Dark matter/energy.

Dark matter... Physicist are awesome.
The sum of all dark matter/energy and detectable matter/energy equates to the total energy of our universe.
The total energy of the universe is precisely zero.
A universe can begin from nothing.
If you have nothing in quantum mechanics, you will always end up with something.
The longer, but far more eloquent source (which I have obviously plagiarized):
Let the great pondering commence (You can also check this out, but remember to keep breathing while the magnitudes destroy your brain).













