It has been over ten years since I read this book (and I believe that it was an old book even then), so the title, and many of the details, completely escape me.
In this novel, the main characters consist of a crew of a ship whose sole purpose is to make first contact with new alien species and learn/teach their language, so that they can establish regular communication norms with that species.
The crew discovers, somehow, that dark matter is made of a bunch of gigantic sentient, hyper-intelligent single-celled organisms. They communicate with them.
Even though we can't detect dark matter, the book contrives a way for them to communicate with the humans. To the best of my recollection, what happens is that the cells are so massive that their gravity traps errant charged particles. They can then vibrate those particles somehow, and the resulting electromagnetic radiation is what the humans detect.
Much of the book is the explanation of the process by which they attempt to communicate. They start with what they consider to be the most universal language: basic arithmetic. I don't remember how it proceeds from there, but I remember the explanation of the process of first-contact communication being a central part of the book.
The species is single-celled and hyper-intelligent. That means each individual is a cell, and you can hold a high level, thought-provoking conversation with one. They also refer to each other as individuals, and have identities for each of them. (Much like we have names for each other.)
I don't exactly remember the size scale of each cell. But at the very least, they are planetary in size. That is, one individual is at least the size of a planet. They may be as big as stars, though I don't recall.