system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that dark matter is real, and not a filler for our lack of understanding of physics?
gpt
This has been asked and answered many times before. 

It is just a filler, we know we have a lack of understanding. But we also know that whatever is missing has to have certain properties, and we call the stuff with those properties dark matter.