I know the facts about the incident, and I want to ask my friend: 'Did you know' or 'Do you know' about the incident?
Which version—'Did you know' or 'Do you know'—uses correct grammar?
If you want to know if your friend knew at the time, you speak in the past tense. If you want to ask your friend if he or she knows at the moment, then speak in the present tense.
In the literal sense (i.e. asking about the other person's knowledge) the past tense (did you know) asks if the person knew in the past and the present tense (do you know) asks if they know now.
Both phrases are also commonly used idiomatically to introduce a new fact without really asking if it was known. In this sense, both past and present may be used interchangeably.