Did Aragorn know about the Balrog in Moria?

[Artanis]

On the quest, Gandalf wanted the Fellowship to go through the mines, but Aragorn was against it. He warned Gandalf specifically:

'I will follow your lead now – if this last warning does not move you. It is not of the Ring, nor of us others that I am thinking now, but of you, Gandalf. And I say to you: if you pass the doors of Moria, beware! '

Why would he say that, if he didn't knew?

Aragorn also mentions that he had been in Moria before. On which occasion was that? Did he learn about the danger at that time?