Estimating the space required to cram every particle of the universe into a single corner is a fun thought experiment that highlights just how empty the cosmos really is. While the exact numbers are uncertain, we can arrive at a reasonable figure by making three well‑justified assumptions.
Assumption 1 – Size of the Universe: For this exercise we treat the observable universe as a cube 30 billion light‑years on each side. That gives a total volume of approximately 2.7 × 1031 cubic light‑years.
Assumption 2 – Total Mass: Current cosmological estimates place the mass of the observable universe at about 1.6 × 1060 kg. See NASA’s NASA page and the “Extension, Age and Mass of the Universe” article for the various methods used to derive this figure.
Assumption 3 – Final Density: If we could somehow prevent the matter from collapsing into a black hole, we could spread it out at the same average density as the Sun. The Sun’s mean density is roughly 1,410 kg/m3 (for comparison, water is 1,000 kg/m3).
Dividing the total mass by the Sun’s density gives a volume of about 1.1 × 1057 m3 for all the universe’s matter.
Since one cubic light‑year contains roughly 1 × 1048 m3, this volume translates to about 1.1 × 109 cubic light‑years. Packing that into a cube yields a side length of approximately 1,000 light‑years.
In other words, if all cosmic matter were condensed into a single corner, it would occupy a cube only a thousand light‑years across—an infinitesimal fraction of the universe’s total volume. In fact, only about 4.2 × 10–18 % of the observable universe would contain any matter at all.
In this context, matter refers to all baryonic and non‑baryonic particles that make up the visible and dark components of the universe—everything from protons and neutrons to dark matter and radiation.
Explore more about the universe’s size, mass, and density on NASA’s website and physics resources such as Magnitudes of Physics.
