user133967: 
I'm wondering what the portion between the block header and the transaction data (the un-annotated section) is. I can't seem to find any information on it.

user13866: The diagram is wrong
The data in the gap between the two brackets is the count of transactions and most of the first coinbase transaction.
The brackets are positioned incorrectly.

Various components of a raw block
That is bitcoin block 170 with Block ID 00000000d1145790a8694403d4063f323d499e655c83426834d4ce2f8dd4a2ee
The block header is, as your image shows,
0100000055bd840a 78798ad0da853f68 974f3d183e2bd1db 6a842c1feecf222a 00000000ff104ccb 05421ab93e63f8c3 ce5c2c2e9dbb37de 2764b3a3175c8166 562cac7d51b96a49 ffff001d283e9e70
Which can be explained as




hex data
meaning




01000000
version=1


55bd840a78798ad0 da853f68974f3d18 3e2bd1db6a842c1f eecf222a00000000
hash of prior block (i.e. ID of prior block)


ff104ccb05421ab9 3e63f8c3ce5c2c2e 9dbb37de2764b3a3 175c8166562cac7d
merkle root


51b96a49
timestamp


ffff001d
bits (target)


283e9e70
nonce




the 02 following the header tells us the block contains two transactions
The first is the coinbase transaction with TXID b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082
Its raw data is
0100000001000000 0000000000000000 0000000000000000 0000000000000000 0000000000ffffff ff0704ffff001d01 02ffffffff0100f2 052a010000004341 04d46c4968bde028 99d2aa0963367c7a 6ce34eec332b32e4 2e5f3407e052d64a c625da6f0718e7b3 02140434bd725706 957c092db53805b8 21a85b23a7ac6172 5bac00000000
The second is a transaction with TXID
f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16
Its raw data is
0100000001c997a5 e56e104102fa209c 6a852dd90660a20b 2d9c352423edce25 857fcd3704000000 004847304402204e 45e16932b8af5149 61a1d3a1a25fdf3f 4f7732e9d624c6c6 1548ab5fb8cd4102 20181522ec8eca07 de4860a4acdd1290 9d831cc56cbbac46 22082221a8768d1d 0901ffffffff0200 ca9a3b0000000043 4104ae1a62fe09c5 f51b13905f07f06b 99a2f7159b2225f3 74cd378d71302fa2 8414e7aab37397f5 54a7df5f142c21c1 b7303b8a0626f1ba ded5c72a704f7e6c d84cac00286bee00 00000043410411db 93e1dcdb8a016b49 840f8c53bc1eb68a 382e97b1482ecad7 b148a6909a5cb2e0 eaddfb84ccf97444 64f82e160bfa9b8b 64f9d4c03f999b86 43f656b412a3ac00 000000

Related:

In what format does a block store the transaction data?
What Are The Parts Of A Bitcoin Transaction In SegWit Format?


133967: <p><a href="https://i.stack.imgur.com/I50pk.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/I50pk.jpg" alt="enter image description here" /></a></p>
<p>I'm wondering what the portion between the block header and the transaction data (the un-annotated section) is. I can't seem to find any information on it.</p>
 13866: <h2>The diagram is wrong</h2>
<p>The data in the gap between the two brackets is the count of transactions and most of the first coinbase transaction.</p>
<p>The brackets are positioned incorrectly.</p>
<hr />
<h2>Various components of a raw block</h2>
<p>That is bitcoin block 170 with Block ID <code>00000000d1145790a8694403d4063f323d499e655c83426834d4ce2f8dd4a2ee</code></p>
<p>The block header is, as your image shows,<br />
<code>0100000055bd840a</code> <code>78798ad0da853f68</code> <code>974f3d183e2bd1db</code> <code>6a842c1feecf222a</code> <code>00000000ff104ccb</code> <code>05421ab93e63f8c3</code> <code>ce5c2c2e9dbb37de</code> <code>2764b3a3175c8166</code> <code>562cac7d51b96a49</code> <code>ffff001d283e9e70</code></p>
<p>Which can be <a href="https://en.bitcoin.it/wiki/Protocol_documentation#block" rel="nofollow noreferrer">explained</a> as</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>hex data</th>
<th>meaning</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>01000000</code></td>
<td>version=1</td>
</tr>
<tr>
<td><code>55bd840a78798ad0</code> <code>da853f68974f3d18</code> <code>3e2bd1db6a842c1f</code> <code>eecf222a00000000</code></td>
<td>hash of prior block (i.e. ID of prior block)</td>
</tr>
<tr>
<td><code>ff104ccb05421ab9</code> <code>3e63f8c3ce5c2c2e</code> <code>9dbb37de2764b3a3</code> <code>175c8166562cac7d</code></td>
<td>merkle root</td>
</tr>
<tr>
<td><code>51b96a49</code></td>
<td>timestamp</td>
</tr>
<tr>
<td><code>ffff001d</code></td>
<td>bits (target)</td>
</tr>
<tr>
<td><code>283e9e70</code></td>
<td>nonce</td>
</tr>
</tbody>
</table>
</div>
<p>the <code>02</code> following the header tells us the block contains two transactions</p>
<p>The first is the coinbase transaction with TXID <code>b1fea52486ce0c62bb442b530a3f0132b826c74e473d1f2c220bfa78111c5082</code></p>
<p>Its raw data is<br />
<code>0100000001000000</code> <code>0000000000000000</code> <code>0000000000000000</code> <code>0000000000000000</code> <code>0000000000ffffff</code> <code>ff0704ffff001d01</code> <code>02ffffffff0100f2</code> <code>052a010000004341</code> <code>04d46c4968bde028</code> <code>99d2aa0963367c7a</code> <code>6ce34eec332b32e4</code> <code>2e5f3407e052d64a</code> <code>c625da6f0718e7b3</code> <code>02140434bd725706</code> <code>957c092db53805b8</code> <code>21a85b23a7ac6172</code> <code>5bac00000000</code></p>
<p>The second is a transaction with TXID
<code>f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16</code></p>
<p>Its raw data is<br />
<code>0100000001c997a5</code> <code>e56e104102fa209c</code> <code>6a852dd90660a20b</code> <code>2d9c352423edce25</code> <code>857fcd3704000000</code> <code>004847304402204e</code> <code>45e16932b8af5149</code> <code>61a1d3a1a25fdf3f</code> <code>4f7732e9d624c6c6</code> <code>1548ab5fb8cd4102</code> <code>20181522ec8eca07</code> <code>de4860a4acdd1290</code> <code>9d831cc56cbbac46</code> <code>22082221a8768d1d</code> <code>0901ffffffff0200</code> <code>ca9a3b0000000043</code> <code>4104ae1a62fe09c5</code> <code>f51b13905f07f06b</code> <code>99a2f7159b2225f3</code> <code>74cd378d71302fa2</code> <code>8414e7aab37397f5</code> <code>54a7df5f142c21c1</code> <code>b7303b8a0626f1ba</code> <code>ded5c72a704f7e6c</code> <code>d84cac00286bee00</code> <code>00000043410411db</code> <code>93e1dcdb8a016b49</code> <code>840f8c53bc1eb68a</code> <code>382e97b1482ecad7</code> <code>b148a6909a5cb2e0</code> <code>eaddfb84ccf97444</code> <code>64f82e160bfa9b8b</code> <code>64f9d4c03f999b86</code> <code>43f656b412a3ac00</code> <code>000000</code></p>
<hr />
<p>Related:</p>
<ul>
<li><a href="https://bitcoin.stackexchange.com/q/113631/13866">In what format does a block store the transaction data?</a></li>
<li><a href="https://bitcoin.stackexchange.com/q/113697/13866">What Are The Parts Of A Bitcoin Transaction In SegWit Format?</a></li>
</ul>
