My understanding was that it is in fact incredibly hot where the gas is, and it is emitting lots of x-rays. And you're jumping ahead of yourself to say that it is matter, we just know that there is gravitational lensing.
My main issue is that the bullet is anomalous in multiple ways, and dark matter doesn't explain the heat issue. If it did, then it wouldn't be useful as an explanation for the rotation curve problem in other galaxies. I feel that a more parsimonious explanation would cover both the lensing and the heat.
You're right. Collisional matter that dissipates radiatively completely explains the heat issue: many of the individual components of the dust and gas in the galaxy clusters came close enough to each other to interact electromagnetically, trading off their momentum-energy for momentum-energy in photons and other products of scatterings. Ordinary intergalactic gas and dust from the two clusters got squashed together and the squashing made it hot enough to glow.
In the standard cosmology (\Lambda-CDM), CDM is cold dark matter that is collisionless, does not dissipate radiatively and does not feel electromagnetism even in very close quarters. So it does not heat up, and its momentum is only influenced the gravitation sourced by the components of the colliding galaxy clusters (which includes the dark matter itself), with the result that the "clouds" of dark matter in each cluster sail right through one another and past all the ordinary matter, with effectively no deflections of their trajectories.
The result is that gravity (measured by lensing of background objects) points to a hot cloud of dust left in the middle of the collision, the stars and other denser visible ahead of that debris, and two mostly-transparent regions leading the way ahead of the visible matter.
> the bullet is anomalous in multiple ways
It's a bit more accurate to say that there are questions still unanswerable by observation of the bullet cluster alone, and that searches for other cluster collisions are likely to provide further partial answers.
In particular, further observations will favour or disfavour different numerical simulations of large scale structure formation under the standard cosmology. However the variables most directly tested align roughly with whether cold dark matter is almost wholly particles similar to heavy sterile neutrinos or almost wholly particles like axions, rather than whether particle cold dark matter is there at all. Even TeVeS proponents aren't especially optimistic on that latter point (e.g. Angus and Diaferio, two prolific TeVeS-as-relativistic-MOND researchers, in their 2012 paper https://arxiv.org/pdf/1206.6231.pdf starting at the bottom of page 23).
Ah, ok, I was going to say, if the excess energy in heat was sufficient to explain the gravitational lensing, you'd have several problems. You're just pointing out that there are other significant anonymous things going on we don't understand. Until they are also explained, the solution is not a complete one.
My main issue is that the bullet is anomalous in multiple ways, and dark matter doesn't explain the heat issue. If it did, then it wouldn't be useful as an explanation for the rotation curve problem in other galaxies. I feel that a more parsimonious explanation would cover both the lensing and the heat.