Question about vacuums (Physics)?

A vacuum to a physicist is totally void of any REAL particles. Technically, all fields in a vacuum have to be at their ground state, which is the lowest level they can go (generally the lowest energy is zero). However, a vacuum is a quantum system (like everything else) and as such, is subject to the energy-time version of the uncertainty principle. This means the vacuum cannot remain at zero energy and fluctuates with something called virtual particles. Although a bit of a misnomer since they are not real particles (they can only temporarily exist ) but they do represent a non-zero contribution to the vacuum.

Just to make the universe more interesting, there is the possibility that the universe is in a false vacuum, meaning that we are stuck in what is called a local minimum and there is a lower energy vacuum possible. That creates the possibility that the universe can 'tunnel' from the false vacuum to the real vacuum at any point in the universe. The unfortunate consequence of that will be the destruction of the universe since it will screw up the stability of matter.

Photons travel through a vacuum at light speed. Yet a photon can exhibit what appears to be mass.

So from modern consideration of what a photon is, photons in a vacuum mean particles in a vacuum.

But they are transitory. A really good vacuum can still have particles other than photons within it, especially a "man- made" vacuum.

to be honest, this is a question that you will never get an answer to.

the reason being that no one actually knows,

there was this amazing video that I can't find, they fill a tube with mercury removing all air, then turn it upside down that the weight of the mercury creates a air pocket, but scientists have no idea what is in there, because since there is that void there has to be something in there, something for the light to pass through etc.

Hmm! Nope