Malekith took a blast from He-El's arm cannon, but he was not sent flying out into Asgard's space.
Asgard was a true flat-earth world.
Asgard was not a sphere like Earth or the Moon, but a relatively flat, celestial-body-like continental mass suspended in space.
Asgard neither rotated on its axis nor revolved around the Sun.
Unlike Earth, whose gravity radiated from the center of the sphere, Asgard's gravity radiated from a point or object beneath the floating continent.
Thus, Asgard had a top that people could stand upon, and a bottom they could not. From there, they would fall through space toward the source of gravity.
Some force prevented the underside and edges of Asgard's continent from eroding away. It kept bodies of water at certain borders from pouring into the void and prevented Asgard's atmosphere from escaping.
Asgard's atmosphere could be understood as a stagnant layer that protected Asgard from radiation beyond the cosmos. This layer had far more tangible substance than Earth's atmosphere.
This was also part of the tremendous difference between Asgard and planetary worlds.
The stagnant layer was Asgard's atmosphere, and beyond it was outer space.
Within the middle of the stagnant layer, one would not fall, but float as though in a Zero Gravity environment. Only its lower layer was affected by the continent's gravity and would pull things downward.
After being blasted away by He-El's arm cannon, Malekith was hurled directly into the middle layer of the stagnant zone, where he floated weightlessly in agony, his body twisted into a savage shape.
Malekith was in a miserable state. The skin had been stripped from his entire face, exposing charred flesh, fascia, muscle, and a blackened skeletal frame. He had taken He-El's arm-cannon blast head-on without any defense, and his physical body was nowhere near