1. The problem is to write $5^{-3}$ as a fraction in simplest form without indices. 2. Recall the rule for negative exponents: $a^{-n} = \frac{1}{a^n}$ where $a \neq 0$ and $n$ is a positive integer. 3. Applying this rule, we have: $$5^{-3} = \frac{1}{5^3}$$ 4. Calculate $5^3$: $$5^3 = 5 \times 5 \times 5 = 125$$ 5. Substitute back: $$5^{-3} = \frac{1}{125}$$ 6. The fraction $\frac{1}{125}$ is already in simplest form. Final answer: $$\boxed{\frac{1}{125}}$$