Why do scientists think 85% of matter in the universe is dark matter? Much of the universe is not observable, so isn't this an assumption?

The dark matter must be in the observable universe if it exists at all. Otherwise we wouldn't know about it.

The "ubobservable universe" if it exists, is by definition unobservable. Hence it would not cause any effect whatsoever on the observable universe, and there would be no anomaly observed which requires the presence of "dark matter" or some other explanation.

There are several videos on youtube to explain that. I'm guessing at least one of them is by "Sabine Hossenfelder"

If they can't see it or proves it exists then it don't.

Now back to the strippers....n my beer.

Because galaxies spin much too fast for the visible matter to hold them together gravitationally.

Therefore, there must be some unseen mass in them.

Much of astronomy is extrapolation, not necessarily assumptions.  Or in other words, they don't guess, they make inferences based on observable data.  Dark matter can't be seen and isn't defined but it is inferred to be present because of the observable motions of celestial bodies.  Or in other words things don't move the way they should if there were no dark matter.  "Dark matter" is however undefined and is a made up term to account for the extra matter needed for celestial bodies to move the way they do.

They call it 'dark matter' because they know it is there but they can't directly observe it. They are not saying it is matter that happens to be dark in colour. It is called 'dark' because it can't be seen.