match() to get the offsets of all matches of a pattern in a string

I have a string of rows of dashes alternating with spaces,
where the number of dashes/spaces is variable

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I want to get a List of the offsets of all the space sequences,
kind of a match() with /g (that's the best I can do to describe
it...) In the case above (ignoring the indent, I would get
[6, 16, 20].

It's the usual problem: I don't know the right :help keyword

/bpj

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