Its probably the anti reflective coating that is scratched or chipped. Could be from anything, your belt buckle, seat belt buckle, phone charging cord, desk, door door frames etc.
Don't be too obsessed about it, I know to you it looks like a massive crater on your crystal. But to the rest of us, had to zoom in to the image to see it.
Wear the watch in good health, you will soon forget about it. If it bothers you that much, take it to a watchmaker to get the coating removed.
Me personally, I would just leave it and enjoy wearing the watch even more.