Answer:
0.000 000 000 000 000 000 000 000 000 01 g/cm³
or
10 fg/km³
Step-by-step explanation:
10^-29 is 1 in the 29th decimal place to the right of the decimal point.
An appropriate set of SI prefixes can be chosen so that the number isn't so difficult to write accurately. Using femtograms per cubic kilometer, we find that the equivalent number is ...
... (10^-29 g/cm³)×(1 fg/(10^-15 g))×(1 cm/(10^-5 km))³ = 10^(-29-(-15)-3·(-5)) fg/km³
... = 10 fg/km³
