Before anything, I'd like to say that I'm a layman (non physicist) and english is not my main language; I apologize if my choice of words make me sound rude / arrogant, that is not my intent.
I often watch high level (abstract) Youtube videos (exclusively from PBS Space Time) and read Wikipedia pages about cosmology, and I fail to understand why some models have not been abandoned.
Our models predict that galaxies should have X density, but our observations say that they have Y density. Instead of considering the model falsified, we proposed an almost undetectable and basically unfalsifiable "thing"; dark matter.
Our models predict that the universe should have X age or Y radius, but our observations don't agree with it; we proposed inflation.
Our models predict that the universe should (or shouldn't) be expanding at X rate, but our observations suggest otherwise. We propose another almost undetectable and unfalsifiable "thing", dark energy.
Our models predict that the edge of galaxies should spin with X velocity, but our observations suggest other values; we propose dark halos.
Our models predict that galaxies should move in such and such directions, but our observations suggest there is such thing as dark flow.
I'm fully aware that I'm probably mixing different models, theories and areas of knowledge here, but as a layman I put all those artifacts in the "what is going on with cosmology bucket?".
So, first, why those mismatches between predictions and observations are not enough to consider such models falsified?
Second, can someone please explain to me, in simple words, why are we doing this? Patching theories with "dark stuff", instead of proposing or studying new theories? I read somewhere about theories that can explain some of these phenomena without invoking "dark stuff", like the "janus model"... But since I'm not mathematically proficient, nor french speaking, I couldn't read anything about it.