Dark energy is the thing that is responsible for that acceleration. Not knowing what else to call it, physicists have dubbed this uniform, undiluting, negative-energy stuff “dark energy”. Unfortunately, dark energy and dark matter are difficult to study (hence the names) so it’s tricky to figure out exactly how much of each is around, but of the energy and matter around today roughly 75% is dark energy, 20% is dark matter, and 5% or less is regular matter. Because the amount of dark energy seems to be proportional to the amount of space around, it seems fairly reasonable to say that dark energy is the “energy of empty space”. There are a bunch of theorists and experimentalists running around trying to directly detect and/or describe dark energy, and with any luck they just might do it.