Interesting little detail: Even though the light doesn’t interact directly with the dark matter, so in a sense it just passes through, the light can still be affected by the dark matter indirectly. Because the dark matter does have mass, or at least interacts gravitationally like it has mass, it actually deforms space-time. This deformation can cause light to travel through a longer path than one might expect.

This has been used to create dark matter “maps”, to show where there is more and where there is less dark matter. It also shows up in gravitational lensing.