https://www.scientificamerican.com/article/how-a-dispute-over-a-single-number-became-a-cosmological-crisis/

Toward the end of the 20th century, the standard cosmological model seemed complete. Full of mysteries, yes. Brimming with fertile areas for further research, definitely. But on the whole it held together: the universe consisted of approximately two-thirds dark energy (a mysterious something that is accelerating the expansion of the universe), maybe a quarter dark matter (a mysterious something that determines the evolution of structure in the universe), and 4 or 5 percent “ordinary” matter (the stuff of us—and of planets, stars, galaxies and everything else we had always thought, until the past few decades, constituted the universe in its entirety). It added up.

Not so fast. Or, more accurately, too fast.