It is an after market band from e-bay. I don't recall the name but I may be able to find it again if you are interested.
It seems to work rather well, it's comfortable, and matches very close in color even though for some reason the photo seems to exaggerate a color difference.
Time will tell on durability. I did notice only once when changing to the sport band that one of the tiny rubber pads which assist in holding the end securely in the watch body, was pulling out. I just pushed it back in and it's been fine since.
My 42mm space gray Sport model with the Product Red® sports band. Multi-colored Modular clock face. I have the original black sports band and a green sports band too.
I tried the Apple leather loop band and the styling just wasn't my cup of tea. I would consider the Milanese Loop in a dark color to match the space gray of the watch but the prices are just a bit too high for me. Plus I really like the sports band look.