At the heart of the US retirement industry, underpinning the later-life plans of millions of Americans, is a set of financial products that hardly anyone can tell you a thing about.

No one knows exactly how much money they control. No one can say how it’s all allocated. No single financial regulator is in charge of them. Yet Collective Investment Trusts are now a multi-trillion dollar business to rival mutual funds or ETFs — and are poised to become the backdoor through which more private assets are added to Americans’ retirement savings.