The universe consists of all and only real things. Something is real
if its pattern is manifested in matter-energy embedded in space-time.
This would include things like memes, but implies that there are some
ideas that have referents that don't exist. We can easily conceive
of and talk about things that don't exist and aren't in the universe
(e.g. galaxy-sized orange dolphins).
Truth is a property only of representations (models, assertions,
maps, statements, portraits). The truth of a representation depends
on its meaning. Its meaning depends on its effect. Its effect
depends on the context. Therefore the truth of a representation
depends on the context. Absolute or objective truth exists only
if you take something as a representation of itself. I'm not sure
why anyone would want to do that, but there it is. (At least it
might give you some common grounds for discussion with someone
that insists that absolute truth exists.)