text: how many atoms are in the universe?   the visible universe is 28 billion light years in diameter (since the earth is 14 billion years old)  1 light year = 9.46 × 10^15 meters   the size of an atom .1 nm = 10^-10 m  calculations:  the diameter of the visible universe is  28 x 10^9 x 9.46 x 10^15 m  = 2.65 x 10^26 m  we are now, going to pack atoms side by side and see how many atoms will fit on this diameter of the universe.  given the diameter is 2.65 x 10^26, the size of atom 10^-10 m, there are,  2.65 x 10^26 ----------------- 10^-10  = 2.65 x 10^36 atoms  estimating the max atoms that will fill a sphere of this diameter,  using 4/3 pi*r^3,  = 4/3 (22/7) (2.65/2 x 10^36)^3  = 9.75 x 10^108 atoms  oridinary matter constitutes 4% of the universe.  so we have 4/100 * 9.75 x 10^108  = 3.9 x 10^107 atoms  so one estimate is 3.9 x 10^107 atoms in the universe.