The ending is open for season two.
Since the story plays out of order in true Jean-Marc Vallée style, here are parts of the epilogue worth knowing, even though they technically happen before the Trivia Night reveals. Throughout the season, we get glimpses of testimonies to the police. So now we're back at where we started. Celeste tells them that he fell. The police investigate the case and they sense that everyone is lying, but don't know why.
Flash forward to Perry's funeral, where Celeste and Jane realize that they'll forever be in each other's lives because Ziggy and the twins are really all half-brothers. How Perry will come back and how his mother (aka Meryl Streep) will grieve, are questions we'll have answered in season two.