True style is the distinctive way a writer uses language
Even though you didn't give us the fragment with the comma slice, it isn't really necessary here as we can answer just based on these 4 options here. All options apart from the second one are correct - only the second option doesn't fix the mistake of the comma splice, whereas the other options do but connecting the sentences properly.
Answer:
There are clusters of galaxies, and the space between them is filled with hot gas, which can only be seen as X-rays or gamma rays.
When measuring how much gas there is between those galaxies, scientists realized that the material should be around five times what can be detected. That undetected matter is known as dark matter.
Answer: "a prepositional phrase" .
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