Gravitational waves are ripples in the fabric of spacetime, spreading from sources like circles on water from a thrown stone. When a wave passes a galaxy, its path bends, like water ripples meeting an underwater boulder. Invisible clumps of dark matter inside the galaxy act like a scattering of small pebbles — each causing a tiny perturbation. Scientists calculated that such distortions for gravitational waves in the frequency range of the future space observatory LISA would amount to a few percent — enough for reliable detection. The changes will manifest as slight variations in the wave's amplitude and arrival time: much like if a familiar voice on a recording became slightly quieter or delayed — sensitive instruments will notice the difference.
The strongest effect occurs when the wavelength is comparable to the size of dark clumps (from tens to hundreds of thousands of kilometers). LISA, scheduled for launch in the 2030s, is precisely designed for such wavelengths. The effect is almost independent of the particles' composition — only their gravity matters, so the method works for any dark matter model.
By analyzing the frequency pattern — essentially taking a spectrum of the distorted ripples — astronomers will map the distribution of dark clumps within the gravitational lens. This will reveal structures invisible to ordinary telescopes because they do not glow. Thus, through the barely noticeable ripples on the cosmic pond, we will read the shape of the hidden depths.
The first gravitational lens was observed in 1979 — a double image of a quasar, created by a galaxy along the line of sight.