We experience gravity as an attractive force but that’s just because our experience of it is very limited. In our experience a gravitational field is generated by mass-energy. But Lorentz Covariance requires that it is also generated by pressure. If the vacuum has energy, and we know that it does both by theory and experiment, it must be under tension, and tension is negative pressure and as such generates gravitational repulsion. It is widely belived that this repulsion gave birth to the visible universe. There is even evidence that it is now blasting the universe apart.