Final answer:
The impossible claim about the first stars is that they were mostly made of dark matter, while in reality, they were formed from hydrogen and helium, marking the end of the cosmic dark ages and having shorter lifespans than later stars.
Step-by-step explanation:
One of the claims about the very first stars in the universe is that they were composed mostly of dark matter. This claim would be impossible given our basic picture of the history of the universe. The first stars, known as Population III stars, were formed from primordial gas composed primarily of hydrogen and helium. They lived fast and died young, contributing to the reionization of the universe and ending the cosmic dark ages. Their formation marked a significant epoch as they were responsible for producing the first heavy elements that would become the building blocks for future generations of stars and planets. Therefore, the correct assertion is that they had relatively short lifespans compared to later stars, which was essential for the cycling of gas through various generations of stellar life.