Sabine Hoffensteder had apparently the same intuition as you and set out to develop a theory of dark matter as being anti-gravitating matter. It didn’t work out…
https://backreaction.blogspot.com/2021/11/does-anti-gravity-...
> Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow.
> So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter.
She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)
https://backreaction.blogspot.com/2021/11/does-anti-gravity-...
> Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow.
> So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter.
She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)