The Darkness spell says:
This also suppresses magical light of your darkness spell's level or lower.
Assuming it isn't heightened, this means Darkness is a 2nd level spell.
The rules for Cantrips say:
A cantrip is always automatically heightened to half your level, rounded up.
The rules for Heightened Spells say:
When you heighten your spell, the spell’s level increases to match the higher level of the spell slot you’ve prepared it in or used to cast it.
So a 5th level caster casting Light would cast it heightened to 3rd level, meaning it counts as a 3rd level spell.
Is this to say that a 5th level caster can counteract Darkness (at its base level) with a cantrip?