dirac vaccum is it present everywhere?

Question

Dirac vaccum is an excellent question of symmetry why is the universe symettric and why is it not in some aspects
2)All the the vaccum has dark energy and dark matter does this mean to say that light only travels in a medium or someother....

Answer 1

"medium" in physics generally refers to a substance in which there is a preferred velocity for travel within in which you may be considered stationary with reference to it. The idea of a universal medium of this sort, called the "ether" was disproved by Michelson and Morley about 100 years ago. Media are now though to be restricted to forms of matter (like a gas or solid). The Dirac vacuum or "sea" is a different sort of beast that is "Lorentz invariant", meaning it has no preferred velocity. That does not mean it has no properties at all (it is not "nothing", which has no properties). It is, for example, seething with virtual electrons waiting for something to bump them into a positive energy state where they can be observed. The most appropriate name to give such a state is the one that it has already: vacuum.

Answer 2

No.
Light can travel through vacuum. Infact light travels fastes through vacuum (3*10^8 metres per sec)
Light does not need a medium to travel.