1E0657-56, the "bullet cluster", is a merger with a uniquely
simple geometry. Comparing the long Chandra X-ray
observation with the recent accurate weak and strong
gravitational lensing maps provided the first direct,
model-independent proof of the dark matter existence (as
opposed to any modified gravity theory), and a direct
constraint on the self-interaction cross-section of the dark
matter particles. I will also show results from Chandra
observations of shock fronts and "cold fronts" in the bullet
and other clusters. They can be used for a number of
interesting physical tests, such as constraining the
electron-ion equilibration timescale in astrophysical
plasmas.