What units are used when describing the length of time that the universe has been expanding? (could possibly be multiple)

billions of years

millions of years

light years

light year kilometers

decades

- - "Length of time" is described in units of time.

- - 'Light years' and 'light year kilometers' are both units of length or distance,

so those are not it.

- - "Decades" is a unit of time, but much too short.

Examples: Judaism has existed for more than 300 decades, the USA

has existed for more than 20 decades, and I have existed for more than

7 decades.

The expansion of the universe could be described in units of decades

if you really wanted to, but that would result in a ridiculous number.

It would be about as inconvenient as discussing the expansion of the

universe in units of seconds or minutes.

- - For just about the same reason, "millions of years" is also a too-small unit.

- - When you're talking about stages in the existence of the universe,

the only unit from this list that makes sense is billions of years, or even

multiple billions.