It&#8217;s complicated and cosmologists are not certain.  One of the issues is only being able to see about 4% of the mass in the universe &#8211; the stars, planets, gas and dust.  About 25% of the mass of the universe is Dark Matter.  It&#8217;s &#8220;dark&#8221; because it doesn&#8217;t emit light that enables us to see ... <a title="dark matter, dark energy and the end of the Universe" class="read-more" href="https://mrmackenzie.co.uk/2021/11/dark-matter-dark-energy-and-the-end-of-the-universe/" aria-label="Read more about dark matter, dark energy and the end of the Universe">Read more</a>