Lol, they haven't even found any yet and they made a map?
I guess you got to do something with that grant money
They mapped the distortion effects on visible light thought to be caused by dark matter. They don't know what it is or how it works, but they know _something_ is causing apparent distortion to the light.
(Neither of us is a physicist, but it’s an interesting idea for a short story).
Every civilization that discovers FTL travel contributes to a more rapid expansion of the universe. The older civilizations beg them to stop, but inevitably they fail.
We have observations that match quite well what would be expected if there was additional matter in the universe. We cannot detect this matter though, so the conjecture is that there must be something that interacts gravitationally with the rest of the universe but not in other ways. We call it dark matter for now. We do not yet know what it consists of, there are even competing theories but mapping the effects of it? That we can do.
This approach is not at all unusual. If you want to know about another example from a different field, look no further than genetics.
Gregor Mendel observed rules for inheritance of certain traits in plants and was able to make predictions from that. He discovered genetics. In the 1850-60ies. This was a long time before Crick, Watson and Franklin discovered the structure of the underlying mechanism for inheritance (DNA).
Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility?
In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?
Black holes are indeed[1] a dark matter contender!
Just because the best explanation[2] we have right now is cold dark matter (CDM), doesn't mean physicists are content. They're looking for new ways[3] to probe not just CDM but other dark matter models as well, like primordial black holes[4].
Dark matter is by no means a settled topic.
[1]: https://en.wikipedia.org/wiki/Primordial_black_hole
[2]: http://pirsa.org/20040087/ "Dark Matter: A Cosmological Perspective", nice colloquium talk giving overview of what we know about dark matter.
[3]: http://pirsa.org/20100052/ "Strong gravitational lensing as a cosmological probe: new ways of constraining dark", random technical talk about new way of probing CDM.
[4]: https://arxiv.org/abs/2002.12778 "Constraints on Primordial Black Holes".
"What If (Tiny) Black Holes Are Everywhere?" https://www.youtube.com/watch?v=srVKjWn26AQ
Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)