Sight, hearing, touch, taste, smell, body awareness (formally, proprioception): six of the widely recognized senses in our bodies that help tell us about the world around us. Yet we have other senses as well. And now meet our surprising latest detector: the immune system. What's that you say? The anatomy textbooks show that the brain and the immune system are almost completely isolated from each other? I thought so, too. But, as usual, researchers probing the world have turned up some fresh insights about how things work.

In our cover story, “The Seventh Sense,” neuroscientist Jonathan Kipnis describes the relationship between the nervous and immune systems. “Mounting evidence indicates that the brain and the immune system interact routinely, both in sickness and in health,” he writes. The immune system may “qualify as a kind of surveillance organ that detects microorganisms in ... the body and informs the brain about them, much as our eyes relay visual information and our ears transmit auditory signals.”

So science giveth, but sometimes it also taketh away. For instance, “Is Dark Matter Real?” ask theoretical physicist Sabine Hossenfelder and astrophysicist Stacy S. McGaugh. This invisible type of matter is thought to accompany the normal matter in the universe to explain how stars orbit in galaxies and how galaxies move in clusters. But astrophysicists have made numerous observations that are difficult to explain with theories about dark matter. Perhaps there's more to gravity than Einstein taught us? The idea that gravity may need to be modified is not widely held, but like the once unquestioned belief in the separation between the brain and immune system, maybe the area is worth a second look.

Sustainable Cities

Usually I’m based in one of the world’s great cities, New York. But around when this issue appears, I’ll be halfway around the planet in the city-state of Singapore to co-emcee an important event run by Scientific American’s parent company, Springer Nature: “Science and the Sustainable City.” Co-located with the World Cities Summit, which draws 20,000 government and business leaders focused on making cities more livable and sustainable, this meeting brings together global experts in academic research, policy and business to discuss and collaborate on solutions. Cities—which already house more than half of the world’s population, rising to two thirds in the next few decades— form crucibles combining both pressing challenges and exciting opportunities. They can be focal points of extreme human stress but also of strong community identity and innovation. And they enable the exploration of new ideas that are impossible to implement in the short term at a national level.

The symposium is part of our Grand Challenges publishing program, which you can learn more about at https://grandchallenges.springernature.comM.D.