God created the world in 7 days. Define "Day"

[theophilus]

From the Book of Exodus

20:10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.

Genesis 1:1 says,

In the beginning, God created the heavens and the earth.

Genesis 1:1 speaks of heavens, plural, while Exodus 20:11 speaks of heaven, singular. The third heaven is mentioned in 2 Corinthians 12:2,3

I know a man in Christ who fourteen years ago was caught up to the third heaven—whether in the body or out of the body I do not know, God knows. 3 And I know that this man was caught up into paradise—whether in the body or out of the body I do not know, God knows—

The heaven God made during the six days is mentioned in verses 6 to 8:

And God said, “Let there be an expanse in the midst of the waters, and let it separate the waters from the waters.” And God made the expanse and separated the waters that were under the expanse from the waters that were above the expanse. And it was so. And God called the expanse Heaven. And there was evening and there was morning, the second day.

The word "earth" has two meanings in English. It can mean the planet we live on or it can mean land. It seems to have both meanings in Hebrew as well. Verse 2 says

The earth was without form and void, and darkness was over the face of the deep. And the Spirit of God was hovering over the face of the waters.

Here it is obvously speaking of our planet. But verse 10 says,

God called the dry land Earth, and the waters that were gathered together he called Seas. And God saw that it was good.

This is what the verse you quoted from Exodus is referring to. You could paraphrase it to say the God created the air, the land, and the seas and everything in them. There is no reference here to the rest of the universe.