In the Bullet Cluster, we have two well defined galaxy clusters that exhibit gravity lensing. Between them is a mass of gas (mostly hydrogen and helium) that is roughly eight to nine times the mass of the two galaxies. The mass of gas doesn't exhibit any lensing. My question is, why do we expect it to act like a lens? For a lens to work, no matter how massive, it needs to focus the rays of light coming from behind it. Since this cloud could be shaped like a box, or a cone or have any shape at all, why are we so convinced that it can focus light? There's no question it can bend light, the question is how is it able to focus light.
A glass marble and a class cube can have the same mass. One will act like a lens, the other won't.
Edit: The answers devolved into a discussion of ΛCDM that I wanted to avoid. Let me rephrase the question: We would expect a spherical, relaxed X-Ray cloud (without significant stellar matter) to exhibit some form of gravitational lensing. Is there any reason that after going through a collision like the Bullet Cluster, the same cloud would display the same kind of lensing (on the scale of one or two Mpcs) or would it just scatter the light in an incoherent way?