Meaning: There are many things about the universe that we do not yet know.
Choice of Words: Universe: everything that exists; Holds: contains; Many: a large number; Secrets: hidden information.
Alternative Expressions
The universe is like a big box of secrets.
Meaning: There are many things in the universe that we don't know about yet, like why stars shine or how black holes work.
Choice of Words: The universe is everything that exists, so it's like a big box. Secrets are things that are kept hidden or unknown, so the sentence is saying that the universe is full of things that we don't know about yet.
The universe is a mystery waiting to be solved.
Meaning: Scientists are always trying to learn more about the universe and how it works.
Choice of Words: A mystery is something that is puzzling or difficult to understand, and a secret is something that is hidden or unknown. The sentence is saying that the universe is like a mystery that we are trying to solve.
The universe is full of surprises.
Meaning: There are always new things being discovered about the universe, and it's always exciting to learn more.
Choice of Words: A surprise is something that is unexpected or unexpected, and the sentence is saying that the universe is full of things that we don't expect.