Final answer:
MACHOs, potential dark matter candidates, are substantial yet compact objects detectable through microlensing. They have not been found in enough numbers to explain dark matter, and research continues to explore other possibilities.
Step-by-step explanation:
Massive Compact Halo Objects, or MACHOs, were a candidate for the elusive dark matter that is theorized to make up a significant portion of the universe's mass. These objects, which are very massive yet compact, could include black holes, brown dwarfs, and white dwarfs. MACHOs are detectable through a phenomenon called microlensing, where they pass between Earth and a distant star, acting as a gravitational lens and temporarily magnifying the star's brightness.
However, observations have not found MACHOs in sufficient numbers within galactic halos to account for the dark matter present. Although searches are ongoing, other dark matter candidates are also being examined, such as red dwarfs and smaller than stellar mass black holes from the Big Bang.