Scanning microscope is used to look at cell structure with 10 nm resolution. A beam of electrons from a hot filament is accelerated to 50,000 km/s and then focussed to a small spot on the specimen. What is the wavelength in nanometers of the beam of incoming electrons?

your answer is, orbit around the nucleus

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 =

3. dark matter

empty space could also be a simple vacuum