Okay, I know it sounds weird, but hear me out:
We know space is expanding, sure. That's been established science for a while now. All three spatial dimensions at the same (increasing) rate.
So ... why should we assume that the 4th dimension -- time -- is static and unchanging like we used to think space was? Could time also be expanding or contracting? (Expanding seems more likely, as it would match what the other dimensions are doing.) After all, spacetime is all one thing, really. Space and time are inextricably linked. When you think of it that way, it seems nearly impossible that space would be expanding while time is not. Spacetime is expanding, so wouldn't that include time as well?
My question here is: what would it look like, subjectively, from our perspective inside it, if time was expanding just like space? Would we be able to measure it at all? Would there be any difference? Could the acceleration of space expansion ('dark energy') actually be explained by time expansion instead?
For a moment, imagine a universe where time definitely is expanding. Even if you don't think time could really be expanding, let's think about that hypothetical universe where it definitely is. What would that universe be like? How would it be different -- if at all -- from our universe?
Does it even matter? If time is expanding, but we still experience it passing at a constant rate, why would we even care whether it's expanding or not? An observer somehow watching it happen from 'outside of time' might, say, see things happening slower and slower ... but for beings living inside of spacetime, with their subjective perceptions also dependent upon the flow of time, would it actually change anything at all?
The biggest problem with measuring any such effect is our frame of reference. All of our measurement tools are stuck in Sol's gravity well, which is itself stuck in the Milky Way's gravity well, and so on.
There's a lot that we don't know, because our viewpoint is limited. For example, the gaps in this chart of observed galaxies:
are caused by all of the objects in the Milky Way which are blocking our view of more distant objects.
We do know that there are a lot of other galaxies around ours, and that they move through space along measurable and predictable paths. Gravity affects time, so time doesn't necessarily progress uniformly everywhere, but at least for the observable universe it must be fairly consistent otherwise we would see strange behavior in the frequencies of light from observed astronomical objects (it would mess with redshift/blueshift). Astronomy relies heavily on redshift/blueshift data, so anomalies would not go unnoticed.