Mr. Darcy left his house a day after Elizabeth had. He went to London and located Mr. Wickham and Lydia.
Darcy tried to convince Lydia to leave Mr. Wickham, but she wouldn't, assuming they would marry at some time or another.
Wickham, however, was definitely not keen on the idea of marrying Lydia, since he was still planning to marry rich.
So, Darcy settled Wickham's debts and gave him a fortune to marry Lydia.
This is all super secret, BTW. And Mrs. Gardiner makes sure to mention what a gentleman Darcy was about the whole thing.
Elizabeth has a lot of feels about this.
More than anything, she's grateful: Mr. Darcy met with a man he despised and bribed him to do the right thing for a girl who Mr. Darcy couldn't respect.
Could he possibly have done it for her?
She goes out walking and Wickham joins her. He fishes around to find out what she knows about his past. Elizabeth admits to him what she knows, but in a genteel manner, and she basically tells him to drop it.