Sounds like he's talking about the bullet cluster... https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster Although that's

Sounds like he's talking about the bullet cluster... https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster Although that's a colliding pair of galaxy clusters. I'm not currently aware of any examples of this effect being observed for just two galaxies.