re: Our universe: An expanding bubble in an extra dimension

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We have known for the past 20 years that the Universe is expanding at an ever accelerating rate. The explanation is the "dark energy" that permeates it throughout, pushing it to expand. Understanding the nature of this dark energy is one of the paramount enigmas of fundamental physics.

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Well, maybe. Since we don't know what dark energy is or where it originates, it's very bold to say that it's the cause of the expansion of the Universe.

Some scientists think that the repulsive force of dark energy overcame the attractive force of gravity 5-6 billion years ago. This caused the runaway acceleration that we see today. The Universe had been expanding since the Big Bang, but gravity had held dark energy in check to some extent until its effect was weakened by distance.

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It has long been hoped that string theory will provide the answer. According to string theory, all matter consists of tiny, vibrating "stringlike" entities. The theory also requires there to be more spatial dimensions than the three that are already part of everyday knowledge.

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In the 1990s, Edward Witten showed that all competing string theories were just different aspects of one theory, now called the Super String Theory. His work has spawned research by others, leading to many ideas about the workings of strings. LINK