Those Are Speed Holes!
There is a newish gaming mouse from Cooler Master, the MM720 which features high end switches and a PixArt PMW3389 sensor capable of sensitivity adjustments from 400 to 16000 DPI. The speed holes on the rear let the RGBs underneath shine through, not to mention the scroll wheel which also spurts out the RGBs. Those cut outs also likely contribute to the light weight of the mouse, a mere 49g.
After you download and install the Cooler Masters MasterPlus+ software suite you will be able to program the six buttons on the mouse, as well as those RGBs and save up to five profiles to switch between on the fly. It is also worth noting that there are two types of switches onboard this mouse, the side buttons are high end Huano physical switches however the left and right buttons use LK optical micro switches.
If you are into lightweight mice, with no easy way of adding more weight, and prefer a claw grip then pop on over to Modders Inc to take a look at the Cooler Master MM720.