Question

Why is it so difficult (for LIGO) to detect intermediate black holes ?​

Answer 1

The problem is that unlike the "light-weight" stellar class and the "heavy-weight" supermassive class, there is no known way to form these intermediate mass black holes. Their existence, which has only been indirectly shown, cannot be accounted for under the existing theory of how black holes form.

Answer 2

Explanation:

or all its vast emptiness, the universe is humming with activity in the form of gravitational waves. Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time, like the clang of a cosmic bell.

Now researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an “intermediate-mass” black hole, with a mass between 100 and 1,000 times that of the sun.

They detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometer-long interferometers in the United States; and Virgo, a 3-kilometer-long detector in Italy.