The Heisenberg uncertainty principle says you cannot measure both the energy of something and its location at the same time, at the quantum level. Therefore, for any point in space you can't be sure of its exact energy level. Since you can specify its location exactly, you can't know how much energy is there, even for an "empty" point in space. It might be very nearly zero, but it's not zero.
This implies that there is a very low level energy field everywhere in space, even supposedly empty points in the vacuum have a small but unknown quantum of energy. This is the vacuum energy. It's near zero, but not zero.
Since this energy exists it has an energy density, which is also near zero, but not zero. Since there's density, there's pressure, like a gas in a volume of space, exerting a pressure. This pressure is near zero, but not zero. And it exists everywhere, although it's nearly zero.
Over very large distances, like the distances between galaxies, this pressure incrementally adds up to a very large force. This force pushes outwards everywhere at once. Over the distances between galaxies this force creates an increasing acceleration. Everything moves further from everything else.
This implication of the Heisenberg uncertainty principle gives rise to the cosmological constant and is the reason why the universe is expanding.
This implies that there is a very low level energy field everywhere in space, even supposedly empty points in the vacuum have a small but unknown quantum of energy. This is the vacuum energy. It's near zero, but not zero.
Since this energy exists it has an energy density, which is also near zero, but not zero. Since there's density, there's pressure, like a gas in a volume of space, exerting a pressure. This pressure is near zero, but not zero. And it exists everywhere, although it's nearly zero.
Over very large distances, like the distances between galaxies, this pressure incrementally adds up to a very large force. This force pushes outwards everywhere at once. Over the distances between galaxies this force creates an increasing acceleration. Everything moves further from everything else.
This implication of the Heisenberg uncertainty principle gives rise to the cosmological constant and is the reason why the universe is expanding.

