I know this! It's like there's a piece missing in the anatomy jigsaw where you can feel what should be there, but isn't; a phallus-shaped void. Sometimes, particularly during arousal, as others have mentioned, it's more acute - it feels like I could touch it if I tried. Finding nothing but empty space brings on dysphoria - always. Looking at that space, even more so.
I didn't realise it was such a common thing.