The universe was created 13.73 billion years ago.  At about 10^-44 seconds after the big bang inflation kicked in and underwent a period of rapid inflation (expansion, this inflation force is thought to be dark energy depicted in Einstein’s lambda term (the cosmological constant) in the right hand side of his field equation describing the energy momentum of the universe.) The cosmological constant is a characteristic of the spacetime fabric of the universe related to its stretching energy (space energy density—commonly referred to as dark energy).  The more the universe expends, the greater this stretching energy becomes.[1]  When the spacetime fabric stretches, the bodies of masses, such as galaxies, move farther apart by the stretching of space.  The cosmological constant is in effect a pulling property that works against gravity.  Since creation, the cosmological constant’s effect has been increasing.  Initial expectations were for the expansion to slow down and for the universe to collapse back in on itself.  For instance, when a ball is tossed in the air its speed slows down and the ball falls to the ground.