I Am Not a Physicist, so I need a real one to explain why I'm wrong, but I've thought for a while the cleanest explanation for dark matter is
1. The universe is inside a black hole.
2. Dark matter is matter which is outside the black hole, which explains why it has mass but no other observable properties.
This is trivially true, btw. The Schwarzschild radius of the observable universe is less than the presumed plausible size of the universe as a whole. Or alternatively, a "black hole" is a region of space that nothing can escape, not even light, and ongoing and accelerating spatial inflation causes that to be true on an even smaller scale -- most of the universe is unreachable by light emitted today.
> Dark matter is matter which is outside the black hole, which explains why it has mass but no other observable properties
Except that it interacts as if it was superimposed over objects in this universe, not from outside of it.
It's a novel idea, but it's just turtles all the way down
The rest of your hypothesis also has no sense, given what we know today. The big "black hole" can't be "outside" of our universe as our universe is all that is, including the space in which it is.
The universe doesn't expand inside of its own space, there is actually more space between the galaxies appearing, that is, the new space is appearing, according to what we observe. So the mechanism can't be easily explained by imagining some mass "outside" as the mass must be in the very space we talk about.
What we see we call the "observable universe" and because of the finite speed of light, observing the stuff in the distance we see the past! What we see in the farthest distance is actually just a glow of the so-called "big bang," a phase of the universe when there were no stars, no galaxies, just the elementary particles, specifically, we see the moment in which that state became "visible", that is, allowed the photons to travel long enough.
So the farthest everywhere we look is just the glow. It is all around because around the point where we are now, just like any other point, there was much less space early enough. The glow is called the CMB:
https://en.wikipedia.org/wiki/Cosmic_microwave_background
It was first time measured in 1964, and the scientists received 1978 Nobel Prize in Physics for that.
We see the big bang in the distance, even if the big bang ended around 14 billion years ago.
I hope you see why we can't explain what we actually observe by imagining the "black hole outside."
> You may have noticed that the universe is actually expanding, rather than contracting as you might expect the interior of a black hole to be. That’s because, if anything, our universe bears a passing resemblance to a white hole. Our universe (according to conventional general relativity) has a singularity in the past, out of which everything emerged, not a singularity in the future into which everything is crashing. We call that singularity the Big Bang, but it’s very similar to what we would expect from a white hole, which is just a time-reversed version of a black hole.
> That insight, plus four dollars or so, will get you a grande latte at Starbucks.
More like it is itself an inside out black hole, or acts like one. I suggest Leonard Susskind's talks/lectures on Youtube for a clear explanation.