What is the cosmological constant?

My question here has to do with the real reason that makes the Universe expand with acceleration. I know very well that the evidence of accelerated expansion are very strong. It seems to me that the main problem of modern physics is to establish the real nature of the so called "dark energy". Is it really a form of energy that makes space expand with acceleration? In particular this unknown form of energy is also assigned a negative sign. I also understand that positive energy makes space to bend positively, and create attractive gravity, whereas negative energy bends the spacetime negatively and creates repulsion. It must have been the same force that made the Universe to expand with acceleration 13.8 billion years ago. At that time negative energy dominated the Universe and hence the Universe expanded with acceleration. Gravity was repulsive not attractive. However, if we go too far into the past, near the Big Bang quantum effects were also important and could initiate a repulsive gravity, despite the supposedly overcondensed state of matter at that time, that would be nothing less than a black hole according to modern theories. How do all of these fit together?