We have good reason to believe that when we talk about dark matter, we really are referring to some kind of &quot;stuff&quot; that has mass and momentum, but doesn&#x27;t interact electromagnetically with normal matter.<p>The most visually striking evidence comes from the Bullet Cluster. We can <i>see</i> two galaxy clusters that have recently collided at ~1% of the speed of light, causing the gas to be slowed and heated. And we can map the mass distribution via gravitational lensing, and see that a large amount of the clusters&#x27; mass has passed right through each other without being slowed. This is more or less what you would predict if &quot;dark matter&quot; is made of weakly-interacting massive particles (WIMPs), and is pretty hard to explain by appealing to a modified law of gravity.<p><a href="http:&#x2F;&#x2F;chandra.harvard.edu&#x2F;photo&#x2F;2006&#x2F;1e0657&#x2F;index.html" rel="nofollow">http:&#x2F;&#x2F;chandra.harvard.edu&#x2F;photo&#x2F;2006&#x2F;1e0657&#x2F;index.html</a><p><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>