Dark matter (matter that has mass but does not interact in any other way) might be the literal solution. But there are also other suggestions (MOND is a big one

Dark matter (matter that has mass but does not interact in any other way) might be the literal solution. But there are also other suggestions (MOND is a big one). The https://en.m.wikipedia.org/wiki/Bullet_Cluster https://en.m.wikipedia.org/wiki/Bullet_Cluster is pretty interesting.