Maybe think of it this way: expecting matter to be easily observable is an anthropocentric point of view, because human intuition defines matter as something that can be observed by the senses.
But the universe was not designed to be observed by us. There's no reason to expect that the majority of matter should be observable.
It turns out that in some ways we actually are precious snowflakes. What we are made off (baryonic matter) makes up less than 5% of the whole "stuff" of the Universe. A mere froth on the surface of existence.
And then some dark model builder says, "Guys, guys. I bet it's SU(3)xSU(2)xU(1) with 3 generations and the parameters tuned so that..."
How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.
Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint.
That's dark matter.
It's obviously bullshit, but it's the 'simplest' explanation.
"Imagine being told there's loads of bugs in your system and it's constantly breaking, and even though no-one can show you a single one of these bugs, tons of users are complaining."
That would fit the situation better: There's only one kind of indication that these things exist.
But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?