_____ constitutes 90% of all matter in the universe and is the lightest element known.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for an element that constitutes 90% of all matter in the universe and is the lightest element known.

2. **Identifying the Element**: The element that fits this description is Hydrogen. It is known to be the most abundant element in the universe.

3. **Hydrogen's Characteristics**: Hydrogen is not only the most abundant element but also the lightest. It has an atomic number of 1 and is represented by the symbol 'H'.

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