Some estimates based on Hubble telescope pictures suggest that there are at least 200 billion stars in our galaxy and that there are more than 100 billion galaxies, all consisting basically of hydrogen and helium plasma [1] . A simple calculation then yields a ballpark figure of about 1050 tonnes for the total 'visible' matter of the universe [2] . Yet, dispersed over the enormous voids of space, this huge mass dwindles to just a few atoms per m3 of space, a density billion times lower than the best vacuum achievable on Earth [3] , while a profusion of radiation passes through 'empty' space in all directions [4] .
Generally, elements heavier than hydrogen and helium make up less than 2 % of a star's mass and the ratio of hydrogen to helium is about 3 to 1. Planets are assumed to account for less than 1 % of the star's mass (e.g., in our solar system they account for only 0.1 %).
The rough calculation of the universe's 'visible' matter (see Sheet) excludes additional matter in the interstellar and intergalactic gas clouds. To grasp the magnitude of the universe's visible matter, you might try to visualize the masses of the oceans, then of the whole Earth (more than 4,000 times the mass of the oceans), then of the Sun (more than 300,000 earths), then of 200 billion suns, and finally this accumulation of suns 100 billion times again. This accumulation of mass supposedly represents only a fifth of the mysterious invisible dark matter.
An ultra-high vacuum, produced in exceptional technological settings, still contains some 1010 molecules per m3, a density 9 orders of magnitude higher than the universe.
Empty space carries an unbelievable amount of information in the form of electromagnetic radiation emanating from all astronomical objects, a portion of which we detect with various types of telescopes.