Scientists think there may be dozens of undiscovered dwarf planets in the Solar system. What I find intriguing is that, we have been able to detect distant galaxies and clusters, locate supermassive black holes...but how is it that dwarf planets in our own solar system are going undetected?
Dwarf planets are observed by the sunlight they reflect. Undiscovered dwarf planets at the outer boundaries of our solar system are simply fainter than the other things mentioned. If they can't be seen then they can't be discovered.
It is possible that they are just about bright enough to be seen but even then, in a single picture they are indistinguishable from the billions of other faint specks of light in the sky.
Finding faint, moving extremely faint objects in a series of pictures is a challenging observational project.
All bright enough dwarf planets in the vicinity of the Sun have already been discovered, for all others only the Keck observatory in Hawaii is able to discover any. Once the Vera Rubin observatory goes into service, it will be able to recognize almost all (dwarf) planets beyond Eris through one double survey of a huge area of the sky.
distant galaxies and clusters, locate supermassive black holes
Galaxies and clusters are very bright objects. That's intuitive: galaxies contain a lot of stars, each of which are roughly as luminous as our sun. Clusters consist of galaxies, so they are naturally even brighter. A faraway object that's very bright is naturally easier to detect than a close object that's very dark (try searching for something in a dark room).
Supermassive black holes are dark, but because they're supermassive, they produce measurable gravitational effects. Most of the time, we know a black exists because of what it does to nearby objects - for example if we observe a star orbiting something small that we cannot detect. In this case the key variable is the mass of the black hole; the larger its mass the easier it is to detect. Supermassive black holes are supermassive, so they are easy to detect. Conversely, there could be a primordial black hole in the Solar System, and we would struggle to detect it.