Step 1: Concept
A function is increasing where its derivative $f'(x) > 0$.
Step 2: Analysis
$f(x) = e^x - x \implies f'(x) = e^x - 1$.
Set $e^x - 1 > 0 \implies e^x > 1$.
Step 3: Conclusion
Since $1 = e^0$, the condition $e^x > e^0$ implies $x > 0$. Therefore, the function is increasing in the interval $(0, \infty)$.
Final Answer: (E)