According to scientists, currently the edge of observable universe from the earth is about 14 billion parsec. This means that the diameter of the universe is nearly 91 billion light years. If we consider all the matter we could see, some say it only makes 4 percent of the matter calculated from the density. What is the universe lacking in such a case. Also which massive things are there which can be observed but not directly seen?
The invisible mass in the universe as you mentioned refers to mass which is not visible using electromagnetic radiation. It is called dark matter and was detected by the virtue of its gravitational effects. The diameter of the universe doesn't play much role in your question.
Secondly, there can be many massive things which can't be seen by EM radiation but can be detected otherwise. Precise details are needed to exactly point out such things.