Final answer:
Deuterium is rare in the universe, and its current abundance helps estimate the density of ordinary matter, which accounts for less than 5% of the universe's critical density. This indicates the presence of dark matter that contributes to the total matter density without affecting deuterium production.
Step-by-step explanation:
Based on current evidence concerning the amount of deuterium in the universe, we can conclude that deuterium is rare in the universe (B). The presence of deuterium provides a glimpse into the early universe’s conditions — particularly its density when the universe was about 4 minutes old. If the initial density was high, deuterium would have transformed into helium, but the fact that deuterium is present suggests that the universe had a lower density, expanding and thinning out rapidly enough for some deuterium to survive.
The measurements of deuterium abundance also help us estimate the current density of ordinary matter in the universe, which is noted to be around 5 × 10-28 kg/m³. This is significantly less than what is required for a critical-density universe, telling us that ordinary matter, including objects like planets and stars, constitutes no more than 5% of the total matter in the universe. Hence, this suggests the existence of dark matter, which does not affect the production of deuterium but adds to the overall matter density.