CMFs - Composite Metal Foams - can stop armor piercing bullets. As in, they turn the bullet into dust on impact. Oh, and you only need an inch of it.
Fun for the military? Right. Except that this stuff is twice as resistant to fire and heat as regular metal and shields well from X-rays, gamma rays and neutron radiation.
So, we have a substance that shields from radiation, resists temperature changes and can take hits.
And is light.
I wonder if it's also airtight...but even if it isn't, we have the outer layer of a long distance spaceship hull right there.