Dark energy comprises about 68% of all the mass-energy of the universe. Dark matter is 32%, with normal matter - everything we are and have experienced - comprising only 4% of all the mass energy of our universe.
To explain the accelerating expansion of the universe scientists have invented dark energy as a repulsive force exerting a negative pressure contributing to Tμν, the stress-energy tensor, and behaving as a force against gravity - causing the universe to expand. It is 68% of all the mass-energy of the universe and yet we really have no idea what it is.
Several leading theories include the vacuum energy of space - space/time expands as a result of particles popping in and out of existence in empty space. Some think an extra-force in addition to the four described by the standard model of physics- gravity, weak and strong nuclear forces, and electromagnetism — the so called "fifth force" - is responsible.
Others have proposed a low-energy field dubbed "quintessence," or fields of tachyons — hypothetical particles that travel faster than light and thus back in time, or that dark energy results from the decay of ‘dark’ matter particles called erebons causing them to emit a form of energy that is driving the expansion, or it is a remnant energy that made it through previous iterations of our universe, among other ideas.
With all of these theories abandoning or at least significantly altering the standard model of physics is required. But there is one explanation that does not.
Dark energy is not energy at all - rather it is geometrical distortion of our space-time by forces lying outside of our universe. Our universe exists in a multiverse and our nearest multiverse neighbors - external universes to us - are pulling our universe outwards in all directions at once.
If we are to follow Occam's razor - of all the theories put forth to date - this seems the simplest. And if true it gives us a window to study the multiverse, something that exists outside our own universe. Pretty cool.
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