Money is Nothing More than what we (our elected representative) assign it. Once upon a time, money was silver and gold coins - that is because it was the acceptable form of exchange, at the time.
Before the creation of Money, we had a barter system.
Even today, we still have small barter systems that goes on.
Money is Nothing more than a medium that facilitate exchanges of goods and services.
Whether we use bitcoin, money denominated in the local currency, USD, EURO, YEN, GOLD - they all can be exchanged for goods and services.
If tomorrow, we see car as being exchangeable for goods and services....it is deemed as money.