system
Given the following post, respond as if you're commenting on Reddit:
human
When Does A Galaxy become a Galaxy?
gpt
There's actually no single definition that can cleanly define a galaxy. 

In the most commonly used definition, a galaxy is a gravitationally bound collection of stars, gas, dust, etc., that lives within a dark matter halo. Indeed, it is considered that under lambda-cdm and hierarchical formation, dark matter haloes collapsed gravitationally first, and then baryons fell (were pulled into) these dark matter haloes. The gas, being collisional, could then form disks and eventually, stars. So a dark matter halo which has within it a collection of stars could be called a galaxy.

However, there are examples of galaxies which appear not to have dark matter. For example, NGC 1052-DF2 and NGC 1052-DF4. It is not yet clear \*why\* these have no DM, but one theory posits they were once part of a normal pair of gas-rich, "regular" galaxies which collided. In such a collision, the DM (being non-interactive) would continue onward, but the gas, being collisional, would collapse in small pockets like beads on a string along the collision path (producing galaxies without dark matter). 

Similarly, there exists a class of galaxy known as a Tidal Dwarf Galaxy. These are formed out of the gas that gets thrown off of galaxies during mergers/gravitational interactions. Because only gas is flung out (not DM), these form systems lacking dark matter, which can get flung out into the general area of the group and eventually become dwarf galaxies. However, these do not follow the formation mechanism listed above and produce some issues, because.... 

Stellar mass is \*also\* not a clean definer. The least massive dwarf galaxies in the literature (\~10\^5 Msun) are less massive than the most massive globular clusters and star clusters (which are definitely \*not\* considered galaxies). 

At the same time, boundedness (or lack thereof) to a more massive system doesn't work either --- while GCs are bound to other galaxies (unless flung free), satellite galaxies are also bound to more massive haloes. Here, people usually cite the fact that satellite galaxies \*do\* have DM haloes while GCs do not. 

All in all, historical precedence (remember that the first galaxies were discovered and "defined" before our understanding of DM and large scale structure formation developed) and a large number of similar-but-different types of systems means we don't really have a completely clean definition. One could imagine setting one, and renaming certain things to no longer be galaxies, but this is unlikely to happen --- we astronomers like to stick to our naming conventions, even once we know they don't necessarily make sense anymore. Anyways, hope this helped provide some context!