Currently this is a mystery. This mystery is what motivated the Dark Energy Survey project. Even though we don’t know what Dark Energy is, we know what it does: it make the expansion of the Universe speed up. If you think of the rate of expansion as the speed of a car – the car was slowing down for about 8 billion years and then “someone” put their foot on the “accelerator” and it started speeding up again. So, for the last 6 billion years the Universe has been getting bigger at an ever increasing rate.
Current estimates are that 70% of the matter-energy budget of our Universe is in the form of Dark Energy. Most of the rest is in the form of Dark Matter.