text: The visible universe is 28 billion light years in diameter (since the earth is 14 billion years old)\n\n1 light year = 9.46 × 10^15 meters \n\nThe size of an atom .1 nm = 10^-10 m\n\nCalculations:\n\nThe diameter of the visible universe is\n\n28 x 10^9 x 9.46 x 10^15 m\n\n= 2.65 x 10^26 m\n\nWe are now, going to pack atoms side by side and see how many atoms will fit on this diameter of the universe.\n\nGiven the diameter is 2.65 x 10^26,\nthe size of atom 10^-10 m, there are,\n\n2.65 x 10^26\n-----------------\n10^-10\n\n= 2.65 x 10^36 atoms\n\nEstimating the max atoms that will fill a Sphere of this diameter,\n\nUsing 4/3 pi*r^3,\n\n= 4/3 (22/7) (2.65/2 x 10^36)^3\n\n= 9.75 x 10^108 atoms\n\nOridinary matter constitutes 4% of the universe.\n\nSo we have 4/100 * 9.75 x 10^108\n\n= 3.9 x 10^107 atoms\n\nSo one estimate is 3.9 x 10^107 atoms in the universe.