Crack on bike wheel rim
It may be worth glancing at this more detailed answer that discussed how aluminum rims were constructed. But in brief, aluminum rims are first extruded in a flat bar, then they’re cut and rolled into hoops, then the ends of each hoop are joined. One method is to simply pin the ends together, and you’re looking at the seam where the ends were joined. Alternatives include welding the ends together. Basically, this is there by design. Look in the corresponding location on the other rim and you should also find a seam.
The seam looks too straight to be a crack. If you hit an obstacle, a more common failure mode would be for the rim’s bead hooks to get dented or to buckle.