How Would Our Universe Be Different Without Dark Energy?

In 1998, two independent groups of scientists both studying the most distant supernova explosions in the Universe reported the same unexpected phenomenon: these brilliant flashes of light, whose intrinsic brightnesses and redshifts were known to great precision, all had a problem, that they appeared to be much fainter than expected. And the higher of a redshift you went to, the greater this problem got. The interpretation? They were more distant — and hence appeared less bright — than the conventional version of the expanding Universe would have predicted. Rather than being filled only with matter and radiation throughout the fabric of space, the Universe also contained this small but important amount of energy inherent to space itself: dark energy.

As our measurements got better and better, and as we accumulated data from other sources as well, like the fluctuations in the Cosmic Microwave Background (CMB) and the clustering properties of large-scale structure, we found out that approximately 68% of the energy in the Universe today was this mysterious dark energy. Yes, there was dark matter, normal matter, neutrinos and radiation all present, and they were all vital to how the Universe expanded and evolved, particularly at early times. But as the Universe aged, dark energy became more and more important, and will eventually approach a full 100% of the energy present within our Universe.