since it contains waves of order you can discern a world by magical power emitted by silver light , bubble world don't have any
The Bubble World that Luna chose when she was reincarnated had a silver light:
"The other day, I found a really beautiful silver bubble. If I go to the bubble world, I want to go there."
"Where is it?"
Luna drew a magic circle and produced a sea chart.
The silver bubble she found is shining.
<Chap 724>
When Anos went outside Militia World, Anos saw Militia World as a bubble with silver light:
The ship moves straight toward that silvery light.
Gradually, little by little, the black sky turns into silver.
As the ship continued to accelerate,
I saw countless bubbles form around me.
"This is...?"
Lay took a breath out of the spectacular sight.
Even Shin couldn't hide his astonishment, as his gaze was stern.
We were in a sea that flowed out silvery light.
Looking back, I could see
an endlessly huge, round bubble of silver light.
Looking with my Magic Eyes, I can certainly see the magical power of the creator god Militia.
This silver bubble was the world we were in.
In other words, we've gotten outside of it.
<Chap 492>
But even if Militia World has silver light, it is still Bubble World and cannot prove the existence of Bubble World:
"The bubble world is a small world that has not yet evolved. In this Ginsui Sacred Sea, it can be said that it is a world that has not been born. This is because the bubble world does not have a main god and a head of state. Without the main god, the order of the small world cannot be maintained. It cannot be completely controlled, and without a leader, the inhabitants of the small world will continue to fight.It is easy to imagine what will happen.''
<Chap 502>
Because it was a bubble world, it would have been impossible to prove the existence of the Militia world.
<Chap 505>