Or, just maybe, there is a fundamental problem with the actual cosmological model.
Just as a quick reminder : post-WWII, we had a standard model that seemed to work pretty well. Fast-forward a few decades, and the improvements to our various measure instruments showed huge discrepancies. To address that, essentially we could either:
A) Admit the error, and back to the drawing board to try and rebuild a new model that fit better with what we could see (no need to restart from scratch, just go back to general relativity, since we've proven it, and try and find out were we've erred from there). Huge task that, among other things, implied that some people who'd got cushy positions from their work on the standard model could expect to see their whole careers go up in smoke.
B) Bolt what amounts to an undefined variable to the standard model, rename the whole mess lambda-CDM, and call it a day. Nothing is solved but hey, it's ok since we've given the discrepancy a funky cool name: dark matter (and later on dark energy).
Is it in any way a surprise to anyone to see that the choice went to B ?