I mean through undisclosed shares or any other financial ways.
It's possible to be a low-key billionaire, sure. Unknown as in not a public figure, of course. Unknown as in no one including the government has any idea what you're worth, no.
There are tons of private companies and partnerships owned by and invested in by hundred millionaires and billionaires and the companies they own. There will be some paperwork somewhere that this person owns 30% of this company which owns 42% of that company, then through the various tax filings of all the companies, should a forensic accountant or auditor so desire, the net worth of an individual could be closely approximated. This would not be public information.
In theory, yes only from a monetary standpoint.
You list shares, but anything public, like shares, will make it next to impossible to be a hidden billionaire.
A theoretical way of being a billionaire that no one knows the identify of is if someone held 1 million bitcoins (or an amount of an object of monetary value, like an amount of diamonds) and the price of bitcoin was over $1000, but no one knew who was behind that bitcoin address, nor did the person behind the address use the bitcoins. One would be a billionaire that no one else would know of ever only if:
- The price of bitcoin (or other monetary object) was high enough in value to make the units of ownership be in the billions.
- The person never used what was owned, as using a bitcoin (or other monetary object) opens the possibility to tracking the identity. How practical is this, though? It's not.
None of that is practical though and what's the point? Plus, the monetary object could crash in value, meaning the billionaire could become a non-billionaire quickly (like Elizabeth Holmes lost most of her net worth after what happened to Theranos).
So it's possible in theory, but it's not very practical.