Systems like the bullet cluster (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bullet_Cluster" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bullet_Cluster</a>) are taken as evidence that most of the matter in large-scale, gravitationally bound astrophysical systems does not behave like luminous matter.