In the voids

How do you see magnetic fields when they're too far away to stick a probe in, and too weak to polarize light? Like, for example, the magnetic fields in the voids between clusters of galaxies...

One thing you can look for is gamma rays from blazars.

It turns out that light can scatter from charged particles, but it can also scatter off other light particles. The effect isn't nearly so strong, but it's there. And if a gamma ray of sufficient energy collides with one of the background microwaves, it can "pair-produce". It might just simply scatter, of course, in which case it loses energy and the microwave photon gains it. But pair-production contributes quite a bit. Now, a positron from that pair-production will often interact with an electron to produce a pair of photons, and the electron scatter off other matter and result in a photon too.

The first thing to notice here is that these new photons have lower energy than the original gamma ray (conservation of energy). The second thing to notice is that they'll be going in more or less the same direction as the electron or positron.

Now in the absence of a magnetic field, those electrons and positrons would keep going in pretty much the same direction as the original high-energy gamma ray. But if there is such a field, it will bend the particles into new directions, and the gamma rays that result from their interactions won't point back to that blazar.

Quanta has a description and picture.

So, when you look at the spectrum of gamma rays from a blazar, some of the low energy gamma rays shouldn't be there. The model say there should be more than we see.

Arguing from an absence isn't very robust--there might be some other reason for the missing gamma rays. The models can be wrong.

But if you look near a blazar, maybe you can see an excess of low energy gamma rays there, bent off into a halo around the star. It looks like you can.

The intergalactic magnetic fields they infer are tiny: of the order 10^-17 to 10^-15 Gauss. That turns out to be big enough to solve another: the universe seems to have expanded faster than you'd expect (this has nothing to do with inflation, which I don't want to try to defend).

So, a little halo of light (well, gamma-ray light) may be telling us something about a region we'll never be able to visit.