Dark matter can be described as matter that
a. does not exert gravitational force but can be directly detected with light
b. cannot be directly detected and does not emit or absorb light
c. exerts gravitational force and makes up most of the stars and planets as 3% of the universe

that would be because of both gravity and friction. so     c

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =