In science, we can never know what anything IS, we can only know what it DOES. For example, mass is THAT WHICH imparts momentum or inertia. Momentum is the TENDENCY for an object at rest to remain at rest unless acted upon by an external force. Inertia is the TENDENCY for an object in motion to remain in motion unless acted upon by an external force. A force is just a PUSH or a PULL. Matter is THAT WHICH has mass and occupies space. We don't know what space IS, but we do know that space defines location and orientation or direction. We don't know what energy IS, we only know energy is the ABILITY to work. Work is what a happens when a force makes a mass move through space. Movement is a CHANGE of position over time. We don't know what time IS, but we do know that time defines ORDER in the chain of events we call CAUSE and EFFECT, i.e. the CAUSE always precedes the EFFECT in time. So, we see that matter, space, energy and time are mutually dependent on each other for meaning, yet only in as much as we can observe what is happening, not the state of existence per se. Note that cosmology proposes that before the moment of the Big Bang, none of these four existed. It was only after the Big Bang that these four fundamental elements of the universe began to unfold into what we see today. That is, it is not correct to think that before the Big Bang, the universe was a perfect vacuum in space waiting in time for matter and energy to appear. Rather, before the Big Bang, not even space or time were present, and upon the Big Bang, the primordial substance of the Big Bang sprang into existence, then differentiated itself into matter, space, energy and time. According to the theory, before the Big Bang there wasn't even nothing.