Space is a vast stretch of emptiness or rather a stage wherein all the events of the cosmos play out. This space is the universe and the universe is this space. Observable Space stretches out to a distance of 90 billion kilometers and contains every celestial object we have ever known from planets and moons to stars and galaxies and all kinds of electromagnetic radiations. Interestingly this space has features comparable to a trampoline. Einstein said that space can warp and curve in the presence of mass ( this gives us gravity). When there is no mass, space is completely smooth and flat. Now suppose we have the power to remove matter from this universe. First we choose to remove the visible matter. What are we left with? We are left with the dark part (not visible) of the universe i.e. dark matter and dark energy. Now suppose we remove all this as well, what are we left with? Are we left with just empty space? Yes…. Or maybe not…
After we discarded all the matter (both dark and light) from the universe, we are supposed to be left with just the space part of the universe which is empty. Now, we think it is empty, but it is not. The question that remains is, what is left here? To answer that we must shrink to the quantum scale. Here we find that space is not only filled with stuff but is extremely violent as well. This is what you see-
What is going on? Well these wave-like structures that you observe in the picture here , are fluctuations in the quantum field. These energy fluctuations in the quantum field are giving out two particles ( energy gets converted to mass). One of these is a matter particle and the other is an anti-matter particle. These two interact and annihilate each other to release the same energy again which dissipates into the quantum field (therefore the law of conservation of mass is not annihilated).
Now, remember a thing I mentioned before. Einstein said space is smooth and flat in the absence of mass. But here you see all these bizarre formations in space. This is the reason as to why it is extremely difficult to unite two of the greatest theories ever created- the quantum field theory and general relativity. General relativity says space is smooth and flat but Quantum Mechanics says it is rough and violent. The theory which has arrived the closest to uniting these two is the string theory but it is still far from completion. Maybe one day, one of you, who are reading this might unite and form the unified theory of physics.