The cosmological constant<br/> was a vacuum energy (the energy of empty space) that kept gravity from<br/> pulling the universe in on itself, ... A problem with the<br/> cosmological constant is that it is constant, with the same<br/> energy density, pressure, and equation of state over time. Dark energy,<br/> however, had to be negligible in the universe's earliest stages;<br/> otherwise the galaxies and all their stars would never have formed.