&lt;p&gt;This week Science Weekly stares into the universe&#039;s heart of darkness. We&#039;re talking dark matter &ndash; the mysterious, invisible, undetectable stuff that makes up some 20% of the universe. Its presence can only be inferred, because without it galaxies would fall apart and the universe would look very different. &lt;/p&gt;&lt;p&gt;What exactly is it?  &lt;/p&gt;&lt;p&gt;To help us answer this question, &lt;strong&gt;Alok Jha&lt;/strong&gt; spoke to &lt;strong&gt;&lt;a href=&quot;http://star-www.dur.ac.uk/~csf/&quot;&gt;Carlos Frenk&lt;/a&gt;&lt;/strong&gt;, director of the world-renowned &lt;a href=&quot;http://www.icc.dur.ac.uk/&quot;&gt;Institute for Computational Cosmology&lt;/a&gt; at Durham University, where he builds model universes inside supercomputers.&lt;/p&gt;&lt;p&gt;Alok asked him what scientists are doing to find dark matter and what happens if, after every avenue has been explored, it still remains missing.&lt;/p&gt;&lt;p&gt;Subscribe for free &lt;a href=&quot;http://bit.ly/science-weekly&quot;&gt;via iTunes&lt;/a&gt; to ensure every episode gets delivered. (Here is the &lt;a href=&quot;http://bit.ly/sw-xml&quot;&gt;non-iTunes URL feed&lt;/a&gt;).&lt;/p&gt;&lt;p&gt;Follow the podcast on our &lt;a href=&quot;http://bit.ly/sw-twitter&quot;&gt;Science Weekly Twitter feed&lt;/a&gt; and &lt;a href=&quot;http://bit.ly/g-sci-twitter&quot;&gt;receive updates &lt;/a&gt;on all breaking science news stories from Guardian Science. &lt;/p&gt;&lt;p&gt;Email &lt;a href=&quot;mailto:scienceweeklypodcast@gmail.com&quot;&gt;scienceweeklypodcast@gmail.com&lt;/a&gt;. &lt;/p&gt;&lt;p&gt;&lt;a href=&quot;http://on.fb.me/g-sci-fb&quot;&gt;Guardian Science is now on Facebook&lt;/a&gt;. You can also join our &lt;a href=&quot;http://on.fb.me/sw-fb&quot;&gt;Science Weekly Facebook group&lt;/a&gt;. &lt;/p&gt;&lt;p&gt;We&#039;re always here when you need us. Listen back through &lt;a href=&quot;http://bit.ly/sci-weekly&quot;&gt;our archive&lt;/a&gt;.&lt;/p&gt;&lt;div class=&quot;author&quot;&gt;&lt;a href=&quot;http://www.guardian.co.uk/profile/alokjha&quot;&gt;Alok Jha&lt;/a&gt;&lt;/div&gt;&lt;div class=&quot;author&quot;&gt;&lt;a href=&quot;http://www.guardian.co.uk/profile/jason-phipps&quot;&gt;Jason Phipps&lt;/a&gt;&lt;/div&gt;&lt;br/&gt;&lt;p style=&quot;clear:both&quot; /&gt;