Matter in the universe that is undetectable but has mass is called dark matter.

Here's why:

* Undetectable: We can't see dark matter directly, as it doesn't interact with light. We know it exists because of its gravitational influence on visible matter.

* Mass: Dark matter has mass, which means it exerts a gravitational pull. This is how we detect its presence.

Let me know if you'd like more details about dark matter!