The mega-IPOs that have investors so excited this year are coming at an interesting and potentially messy time for markets. Primary markets—i.e. the market for newly issued stocks or bonds—are in an awkward place. SpaceX is expected to raise up to $75 billion in its IPO, which would make it the biggest offering ever. But that money is going to have to come from some other part of the market where investors have already parked their cash. That's because cash on the sidelines is near historic