The answer is that I do not know what the mark of Cain is. Genesis does not tell us, so we have to admit that we do not know. Commentators differ as to the exact nature of this “mark.” Was it an actual physical mark? It may have been. If so, what was it about this mark that caused people to understand that they had better leave him alone? The simple answer is this: We do not know. Might it have been something other than an actual physical sign? Maybe. Maybe God marked Cain in his (ie. God’s) mind and caused people therefore to not persecute him. Either way we know this. Despite his violent past, Cain remained unmolested during his life. Honestly, no one knows, and if God had been concerned that we know he would have told us. God chose to not reveal this information to us, so we would be wise to accept his decision to not tell us and leave it at that (not that there is anything wrong in at least trying to speculate, but…)
I think we would do well to accept the admonition of Deuteronomy 29:29. “The hidden things belong to the Lord our God, but the revealed things belong to us and our children forever, that we may follow all the words of the law.” The nature of this “mark” has not been revealed to us, perhaps because it is not important to how we follow the words of the law, so we probably should be content with this.