You're fishing for "vacuum", but the question shows some uncertainty
with the concepts.
All electromagnetic waves travel at the same speed through vacuum.
That's a fact.
It's also true that they all travel at the same speed through jello, but it's
slower than their speed in vacuum.
The key point is that all electromagnetic waves travel at the same speed,
as long as you're measuring them all in the same material medium.
The speed is highest in vacuum, and some other different, lower speed
in different materials.
