Is there an opposite of LINQ's All method?

The exact opposite of All() is essentially None, but since LINQ has no None() method, you can accomplish the same result through !set.Any().

!a_list.Any(item => item.matches == true)

This will produce true if none of the items in a_list have a matches value that is true.

Another example:

names.All(item => item.StartsWith("R"))

is true if all of the items in names start with R (as you know already).

!names.Any(item => item.StartsWith("R"))

is true if none of the items in names start with R.

Based on your comment below, it sounds like you might just be looking for a way to accomplish the same result as your current code snippet, but in a different way. This should provide the same result as your current code, but without the ! in the predicate:

!a_list.Any(item => item.matches == true)

This can be further simplified to:

!a_list.Any(item => item.matches)

I'd imagine yours could be simplified as well, to this:

a_list.All(item => !item.matches)

There's rarely a good reason to explicitly compare a boolean value with true or false.