https://www.wired.com/story/the-universe-is-expanding-faster-than-it-should-why/

The discrepancy between how fast the universe seems to be expanding and how fast we expect it to expand is one of cosmology’s most stubbornly persistent anomalies.

Cosmologists base their expectation of the expansion rate—a rate known as the Hubble constant—on measurements of radiation emitted shortly after the Big Bang. This radiation reveals the precise ingredients of the early universe. Cosmologists plug the ingredients into their model of cosmic evolution and run the model forward to see how quickly space should be expanding today.