Here's an expanded explanation of the "lots of black holes" hypothesis, posted this week by PBS Space Time.
"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)