Lydia and Wickham arrive at Longbourn. Lydia is giddy over her marriage, mocking her older sisters for failing to get married before she did. Wickham acts as if he did nothing wrong.
Lydia is as shallow and senseless as 4Mrs. Bennet. Wickham is just as crass, in his own way.
Yet during their ten-day visit, Elizabeth observes that Wickham doesn't entirely return Lydia's infatuation. She figures he ran away from creditors in Brighton and, preferring companionship, took Lydia along.
This is a marriage founded not on love, but on opportunity and poor decisions. Since divorces were very uncommon, the relationship will only sour.
While gloating about the details of her wedding, Lydia reveals to Elizabeth that Darcy attended the ceremony. Lydia quickly apologizes: it was supposed to be a secret. Elizabeth burns with curiosity and writes to Mrs. Gardiner for more details.
Darcy is the absolute last person Elizabeth would expect to be at Wickham's wedding. To be polite, Elizabeth can't directly ask about it, furthering the mystery.