Bearing in mind the provisions of the 1st Law of Thermodynamics, which decrees that energy can neither be created nor destroyed, all the energy in the Universe must have been there at the time of the Big Bang. Dark energy began to accelerate the expansion about 5 billion years ago, but in what form was it stored before it mysteriously began to manifest itself?
We don't really know enough about Dark Energy now let alone back then. We're not even entirely sure why there's so much conventional matter versus antimatter, even though logically there should be the same amount of each.
The 1st Law of Thermodynamics is seemingly true for Generalistic observations, but when we look at Quantum levels, even a perfect vacuum has particles and antiparticles (and therefore energy) being created from nothing, before recombining into nothing. So the 1st Law of Thermodynamics isn't explicitly accurate for that scale