Sgr B2 is the most massive gas clump in our Galaxy. It's about 100 pc away from Sgr A*, the black hole at the center of the Galaxy. It is forming stars prodigiously, and has extreme properties in many regards.
In many physical aspects - mass, luminosity, velocity dispersion, temperature - the Sgr B2 region is similar to "proto-cluster clumps" seen in other galaxies, e.g. the Antennae galaxies.
There are plenty of mysteries about Sgr B2, including its extraordinarily diverse chemistry and its peculiar kinematics.
I'm raising a sort of new one. There appears to be a hole next to Sgr B2, towards the southeast in Galactic coordinates. It appears as a relatively empty spot in the dust continuum bands. It doesn't seem to be filled in with anything in particular.
Note the slider here: you can view Sgr B2 at any wavelength where I have data. The green circle highlights the major deficiency that is evident in all dust-containing bands. There are other "minor" deficiencies elsewhere.
It is tricky to interpret this: I don't think the absolute flux densities in any of the millimeter observations are perfectly trustworthy because of their proximity to the extremely bright Sgr B2 point sources.
Additionally, this gap is not perfectly evident in NH3 or CO. In the HOPS NH3 maps, there is a relative lack of emission at ~56 km/s - relative to neighboring regions; this is actually the peak of the line profile extracted from the ellipse!
This is in contrast to the central "Gas Hole" which has been noted by others; it exists in CO and NH3. There is almost certainly some NH3 self-absorption involved. The "Gas Hole" coincides with the peak in other gas species and the peak in the dust emission, so it really seems to be a different phenomenon, having to do with line-of-sight confusion and excitation conditions.
The one feature of the "dust gap" that distinguishes this part of Sgr B2 from the other regions surrounding it is that 8 micron emission can readily be observed here. A little 24 micron emission is in the gap as well, but not as much as the 8 micron. The 70 micron emission is quite weak, but not really deficient. Perhaps in this region, very small grains are more common than in its surroundings?
There's a lot more hypothesizing to be done here.