Coke or Pepsi? Messi or Ronaldo? Taylor Swift or…well, without wanting to set the Swifties against Physics World, let’s just say there’s often a tribal element to who we support or the choices we make.

In the world of cosmology, one heated divide is whether you’re for dark matter or modified Newtonian dynamics (MOND). Both theories attempt to explain the discrepancies between the predicted gravitational effects in the universe and some of the actual observed motions of stars and galaxies.

In the latest episode of Physics World Stories, Andrew Glester speaks to two cosmologists on opposing sides of this debate. Stacy McGaugh from Case Western Reserve University in the US is a former dark-matter researcher who switched sides overnight after MOND successfully predicted the rotation velocities of stars in galaxies.

The other guest, Indranil Banik from the University of St Andrews in the UK, took the opposite journey. While working on a six-year project to measure MOND in wide binaries, he found no deviation from standard Newtonian gravity at all – a hammer blow for MOND. Now a dark matter advocate, Banik cites observations in our own solar system as further evidence against MOND. Naturally, others disagree.

For more detailed insight into this debate, see the recent Physics World feature “Cosmic combat: delving into the battle between dark matter and modified gravity“.