Bond fund managers have so much cash they’re turning to the derivatives market to put it to work, pushing down the cost of protection against defaults close to levels that prevailed when central banks were just starting to raise interest rates.

The bets on tightening default spreads are the latest sign of the overarching optimism that’s enveloped markets, where credit investors flush with cash have been buying up large amounts of new debt and pushing back the so-called maturity wall that was a major source of concern just six months ago. Money managers are using credit derivatives indexes like the Markit CDX North American Investment Grade Index to gain the exposure they want.