A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars. . . .
Early in the 20th century the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the Milky Way. Globular clusters are found in nearly all galaxies. In spiral galaxies like the Milky Way they are mostly found in the outer spheroidal part of the galaxy – the galactic halo.
They are the largest and most massive type of star cluster, tending to be older, denser, and composed of lower abundances of heavy elements than open clusters, which are generally found in the disks of spiral galaxies. The Milky Way has more than 150 known globulars, and there may be many more.
From Wikipedia (star clusters, which are clumps of stars that are smaller than galaxies, are not to be confused with galaxy clusters, which are clumps of many whole galaxies, which together with the "cosmic web" of matter in the universe, are the largest structures in existence in the universe).
We believe that we have a reasonably good understanding of the astrophysical processes that give rise to globular clusters in galaxies. But, globular clusters are observed to be about 23 times less common than simply back of napkin estimates would suggest.
This could be due to "cluster evaporation", due to mergers of globular clusters into other structures or larger globular clusters, or due to other astrophysical processes that have not been accounted for in coming up with this quick estimate of how many of them there should be.
We don't understand the known factors that are in play in astrophysical mass assembly process at this scale well enough to even begin to speculate on any "new physics" effects that could contribute to this discrepancy between theoretical expectations and our astronomy observations.
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