NEW: Retail investors are pouring into U.S. stocks at a record pace. According to Citadel Securities, the day of SpaceX’s blockbuster IPO marked the largest single day of net retail stock buying ever recorded by the firm, which handles roughly 35% of U.S. retail trading volume. Retail activity in both equities and options has surged to unprecedented levels, helping power the broader market rally. SpaceX options also saw strong demand when trading began this week.(Frank Chaparro)