Sister question to How do you communicate with antimatter beings?
Defining dark matter for this question
According to NASA, 27% of the universe is dark matter - and about 5% of the universe is normal matter. Dark matter is a substance that doesn't interact with electromagnetism. That means light passes straight through (it's invisible), and that matter may also be able to "phase" through it (it's intangible and won't take our sound waves, either).
We only know it exists because it interacts with gravity.
For this question, assume that a fifth fundamental force, "dark electromagnetism" exists, and that humans haven't detected it yet. In this world, it allows for "dark light" analogous to ours; it allows "dark elements"; and it allows dark molecular bonds. These maps actually show dark nebula, and some dark solar systems exist in places we haven't looked yet$^1$.
So aliens have arrived in the solar system, and they're made of the stuff.
We detected them$^2$ because their ships have large-scale gravitational effects (lensing, attraction of matter), and they're built with dark handwavium$^3$ that allows them to stay ship-shaped without collapsing into spheroids. They do not know we're here, just our planet, and they have a level of technology parallel to ours - except the giant, interstellar, physics-defying ships.
What are effective ways for our groups to communicate?
How can we get their attention, knowing we can't see each other or hear each other? What could we build or do to allow interaction and conversation between the groups?
$1$ This is my cop-out for the "dark matter doesn't arrange itself into stuff" findings.
$2$ If you have a better explanation please let me know, I don't know how else we'd detect them without answering the question of how we communicate with them.
$3$ Even the crazy physics-turned-on-its-head aliens have handwavium. It's trendy in 2017.