There's was something wrong in the room, but I couldn't quite tell what it was. I felt as if I had entered a house with the gas stove left on; there was something dense and strange in the atmosphere though invisible to my eyes.
afterlater I realized: Anna was no longer in the room.
So instead of writing "seconds after I realized what it was" I just left it as "seconds after I realized (because I already wrote what it was in one of the sentences above).
Is it acceptable to omit what it was in a situation like this?